The S&P 500 list issue today is, by weight, the top 5 to 10 companies dominate. And the top 20 or so is extremely tech heavy.I agree with your idea of investing in yourself and a home first (and your children if you have them).
However I read an interesting article that said the S&P500 is now like 55% owned by index funds - so its no longer really diversified - since you own the exact same 500 stocks in the same proportion as everyone else - so when everyone heads for the door it doesn't matter how diversified the companies are. Got me thinking thats for sure.
The important thing to understand is that the average return over time beats just about everything. It is a true no-brainer.