I keep seeing posts about an upcoming lost decade, but the market keeps rising

People claim that P/E ratios are at levels last seen in March 2000 or 2008. I get it, but the market keeps trudging along.

Doesn’t the market always revert to the mean? Historically it’s done 9-10% per year, and the last decade has kinda looked like the 90s.

What do you think? Are we in a new normal? Or are valuations too rich?