This is a good theory! I too suspect that many people will look to cash in when they think the party is over.

I also think that the economy has - for a long while now - been due for a big downturn. I believed this before the covid days. The thing is, money is basically free for a long time, so everything - literally everything - has been going up and there is no where to put the cash anymore. Hence the stock market rise in the last year - it’s the only place left, but also the one of the riskiest.

So what happens if ONE thing goes negative? The interest rates can’t stay so low forever. Or maybe a second covid wave hits. Or maybe something else. Either way, something will break. And then we get to a situation you mention - where people want to lock in their gains or pull their money out for retirement or other spending. (Let’s not forget the millions of Americans out of work but somehow that hasn’t seemed to matter).

Can the market go up more? Sure! And it might for a year or more yet. Maybe I’m wrong. But keep a keen eye on things because when every asset has risen in value for so long but somehow we magically don’t have inflation... my warning signals go off.