Picking the S&P500 over a world index because you think it will outperform, has the same problem as picking individual stocks over an index. You can't actually know which will outperform in the future.
I live in the UK: if I buy the S&P500 over the FTSE100 (or even more so the 250, the next 250 largest companies which are typically more UK-market-oriented) I'm making a US-weighted bet. But maybe I think I'll move there, and should have that exposure. Or maybe I spend a lot of money all over the world and want a more global exposure overall.
I think at least vast majority index is right for basically everybody, but you do still need to think about which index/indices are most applicable to your situation/intentions.