Here is a simple example of how daily leverage fails, when applied over periods longer than a day. It is specifically adjusted to be more extreme than the actual market so you can see the effects more readily.
You buy a daily leveraged fund and the index is at 1000. Suddenly the market goes crazy, and goes up to 2000 - a 100% gain! Because you have a 2x ETF, you will find your return to be somewhere near 200% (if the ETF did its job). Then tomorrow it goes back to normal and falls back down to 1000. This is a fall of 50%. If your ETF did its job, you should find your loss is somewhere near twice that: 100%. You have wiped out all your money. Forever. You lose. :)
The stock market does not, in practice, make jumps that huge in a single day. But it does go up and down, not just up, and if you're doing a daily leveraged ETF, your money will be gradually eroded. It doesn't matter whether it's 2x leveraged or 8x leveraged or inverse (-1x) or anything else. Do the math, get some historical data, run some simulations.
You're right that it is possible to beat the market using a 2x ETF, in the short run. But the longer you hold the stock, the more ups and downs you experience along the way, and the more opportunity your money has to decay.
If you really want to double your exposure to the market over the intermediate term, borrow the money yourself. This is why they invented the margin account: Your broker will essentially give you a loan using your existing portfolio as collateral. You can then invest the borrowed money, increasing your exposure even more. Alternatively, if you have existing assets like, say, a house, you can take out a mortgage on it and invest the proceeds. (This isn't necessarily a good idea, but it's not really worse than a margin account; investing with borrowed money is investing with borrowed money, and you might get a better interest rate. Actually, a lot of rich people who could pay off their mortgages don't, and invest the money instead, and keep the tax deduction for mortgage interest. But I digress.) Remember that assets shrink; liabilities (loans) never shrink.
If you really want to double your return over the long term, invest twice as much money.