Also, over a span of fifteen years, say 2000-2015, how would a "diversified" portfolio fared vs a portfolio primary comprised of stock index ETFs?
An interesting useful site for this is: https://dqydj.com/finance-calculators-investment-calculators-and-visualizations/#InvestmentCalculators
It contains a number of different calculators that can give you a rough rate of return (accounting for inflation and reinvestment of dividends) for a number of different returns.
According to it, from 1/1/2000 to 1/1/2016, using the S&P 500 calculator (adjusting for inflation, reinvesting dividends), an S&P 500 investment returned 29.216% over that time period, which very much underperformed A-rated corporate bonds at 69.776% - 79.708% for the same period. The S&P 500 investment even underperformed 10-year T-bills at 32.638%.
But you should note that that is a function of the window. If you extend the window from 1/1/2000 to 1/1/2018, the S&P 500 almost exactly ties the high end of the bonds. If instead you advanced the window 2 years (ie, not starting almost immediately preceding a crash), using 1/1/2002 - 1/1/2018, the S&P 500 becomes a clear winner at 140.367% vs the bonds' 50.598% - 61.324%. If you expand the window to 1/1/1989 (roughly the earliest date the site has bond data for) to 1/1/2018, the S&P 500 becomes 783.196% to the bonds' 185.206% - 209.755%.
This is why equities (and index funds in particular) are recommended as long term, not short term, investments.
For your specific question about an index-only vs a diversified portfolio, in the 1/1/2000 - 1/1/2016 window, the diversified portfolio would outperform because of its bond holdings; for 1/1/2000 - 1/1/2018, it would be equivalent, and for the other windows it would underperform. But the point of a diversified portfolio is not to maximize gains, it is to minimize risk, which becomes more and more useful the closer you get to actually withdrawing your investment.