Diversification is the only real free lunch in finance (reduction in risk without any reduction in expected returns), so clearly every good answer to your question will be "yes." Diversification is good." Let's talk about many details your question solicits.
Many funds are already pretty diversified. If you buy a mutual fund, you are generally already getting a large portion of the gains from diversification. There is a very large difference between the unnecessary risk in your portfolio if you only hold a couple of stocks and if you hold a mutual fund. Should you be diversified across mutual funds as well? It depends on what your funds are. Many funds, such as target-date funds, are intended to be your sole investment. If you have funds covering every major asset class, then there may not be any additional benefit to buying other funds.
You probably could not have picked your "favorite fund" early on. As humans, we have cognitive biases that make us think we knew things early on that we did not. I'm sure at some point at the very beginning you had a positive feeling toward that fund. Today you regret not acting on it and putting all your money there. But the number of such feelings is very large and if you acted on all those, you would do a lot of crazy and harmful things. You didn't know early on which fund would do well. You could just as well have had a good feeling about a fund that subsequently did much worse than your diversified portfolio did.
The advice you have had about your portfolio probably isn't based on sound finance theory. You say you have always kept your investments in line with your age. This implies that you believe the guidelines given you by your broker or financial advisor are based in finance theory. Generally speaking, they are not. They are rules of thumb that seemed good to someone but are not rigorously proven either in theory or empirics. For example the notion that you should slowly shift your investments from speculative to conservative as you age is not based on sound finance theory. It just seems good to the people who give advice on such things. Nothing particularly wrong with it, I guess, but it's not remotely on par with the general concept of being well-diversified. The latter is extremely well established and verified, both in theory and in practice. Don't confuse the concept of diversification with the specific advice you have received from your advisor.
A fund averaging very good returns is not an anomaly--at least going forward it will not be. There are many thousand funds and a large distribution in their historical performance. Just by random chance, some funds will have a truly outstanding track record. Perhaps the manager really was skilled. However, very careful empirical testing has shown the following: (1) You, me, and people whose profession it is to select outperforming mutual funds are unable to reliably detect which ones will outperform, except in hindsight (2) A fund that has outperformed, even over a long horizon, is not more likely to outperform in the future.
No one is stopping you from putting all your money in that fund. Depending on its investment objective, you may even have decent diversification if you do so. However, please be aware that if you move your money based on historical outperformance, you will be acting on the same cognitive bias that makes gamblers believe they are on a "hot streak" and "can't lose." They can, and so can you.
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One of your questions is whether it makes sense to buy a number of mutual funds as part of your diversification strategy. This is a slightly more subtle question and I will indicate where there is uncertainty in my answer.
Diversifying across asset classes. Most of the gains from diversification are available in a single fund. There is a lot of idiosyncratic risk in one or two stocks and much less in a collection of hundreds of stocks, which is what any mutual fund will hold. Still,you will probably want at least a couple of funds in your portfolio. I will list them from most important to least and I will assume the bulk of your portfolio is in a total US equity fund (or S&P500-style fund) so that you are almost completely diversified already.
Risky Bonds. These are corporate, municipal, sovereign debt, and long-term treasury debt funds. There is almost certainly a good deal to be gained by having a portion of your portfolio in bonds, and normally a total market fund will not include bond exposure. Bonds fund returns are closely related to interest rate and inflation changes. They are also exposed to some market risk but it's more efficient to get that from equity. The bond market is very large, so if you did market weights you would have more in bonds than in equity. Normally people do not do this, though. Instead you can get the exposure to interest rates by holding a lesser amount in longer-term bonds, rather than more in shorter-term bonds. I don't believe in shifting your weights toward nor away from this type of bond (as opposed to equity) as you age so if you are getting that advice, know that it is not well-founded in theory. Whatever your relative weight in risky bonds when you are young is should also be your weight when you are older.
International. There are probably some gains from having some exposure to international markets, although these have decreased over time as economies have become more integrated. If we followed market weights, you would actually put half your equity weight in an international fund. Because international funds are taxed differently (gains are always taxed at the short-term capital gains rate) and because they have higher management fees, most people make only a small investment to international funds, if any at all.
Emerging markets International funds often ignore emerging markets in order to maintain liquidity and low fees. You can get some exposure to these markets through emerging markets funds. However, the value of public equity in emerging markets is small when compared with that of developed markets, so according to finance theory, your investment in them should be small as well. That's a theoretical, not an empirical result. Emerging market funds charge high fees as well, so this one is kind of up to your taste. I can't say whether it will work out in the future.
Real estate. You may want to get exposure to real estate by buying a real-estate fund (REIT). Though, if you own a house you are already exposed to the real estate market, perhaps more than you want to be. REITs often invest in commercial real estate, which is a little different from the residential market.
Small Cap. Although total market funds invest in all capitalization levels, the market is so skewed toward large firms that many total market funds don't have any significant small cap exposure. It's common for individuals to hold a small cap fund to compensate for this, but it's not actually required by investment theory. In principle, the most diversified portfolio should be market-cap weighted, so small cap should have negligible weight in your portfolio. Many people hold small cap because historically it has outperformed large cap firms of equal risk, but this trend is uncertain. Many researchers feel that the small cap "premium" may have been a short-term artifact in the data. Given these facts and the fact that small-cap funds charge higher fees, it may make sense to pass on this asset class. Depends on your opinion and beliefs.
Value (or Growth) Funds. Half the market can be classed as "value", while the other half is "growth." Your total market fund should have equal representation in both so there is no diversification reason to buy a special value or growth fund. Historically, value funds have outperformed over long horizons and many researchers think this will continue, but it's not exactly mandated by the theory. If you choose to skew your portfolio by buying one of these, it should be a value fund.
Sector funds. There is, in general, no diversification reason to buy funds that invest in a particular sector. If you are trying to hedge your income (like trying to avoid investing in the tech sector because you work in that sector) or your costs (buying energy because you buy use a disproportionate amount of energy) I could imagine you buying one of these funds.
Risk-free bonds. Funds specializing in short-term treasuries or short-term high-quality bonds of other types are basically a substitute for a savings account, CD, money market fund, or other cash equivalent. Use as appropriate but there is little diversification here per se.
In short, there is some value in diversifying across asset classes, and it is open to opinion how much you should do. Less well-justified is diversifying across managers within the same asset class. There's very little if any advantage to doing that.