I personally have used 15 year returns in the past, not because I want to go out of my way to "cherry pick" bad numbers, but because that's when I retired (early 2000). What I actually saw in the 1990s from people who had 401Ks using mutual funds is that people I worked with that had guaranteed investment contracts from insurance companies did better than the people with mutual funds. I actually did better buying a couple of junk bonds and holding to termination.
For my former coworkers, the CAGR for the S&P since 2000, adjusted for dividends and inflation, is 1.91%. No, that's not a misprint and I don't think that's what the mutual fund companies were telling them to expect. Yes, this is not representative of 1984-2000, but as Warren Buffett has pointed out, the Dow also went nowhere from 1969-1984. You are NOT guaranteed any kind of average return like 10%, just by throwing you money at index funds.