DAE find the mantra of buy an index fund and hold it kind of misleading?

Time in the market beats timing the market,

Warren Buffett recommends the best things investors can do is buy the S&P500 and hold it

But doesn’t it feel like of like passive index investing is creating this huge bubble and kind of makes honest valuations driven investing pointless? It feels like everyone just expects the markets to go up in the long run, when we don’t actually have a guarantee of that happening?

And let’s say markets do trend up in the long run. By how much? And what if there is a 30 year period where returns are negative? Isn’t that a significant chunk of anyone’s time horizon?

Did holding European or Japanese indices turn out to be good moves?