Sure, valuations are high, but what else are rich people going to do with their money? Buy bonds? All that money has to go somewhere.

I think you'll find that the largest components, by far, of "that money" (the money in the stock market) is

1) debt (leverage, options, futures contracts of all kinds ... whatever you want to call it). This is by far the largest component, and represents more than 50%

2) pensions (most of which are severely underfunded in the US and catastrophically underfunded, or simply not funded at all, in the EU) (not funded at all means the government bound itself to fulfill pension obligations directly from tax revenue without any investment, and of course just loaded up on obligations)

At this moment it isn't common knowledge, but the big financial news item of the 2018-2019 to 2030 period will be pensions, and what will happen to individual pension funds. I would strongly advise people on a pension to figure out how they can somehow own the house they live in. I guarantee it will make a bigger difference in their quality of life than any investment short of bitcoin.

3) companies themselves

And only after that do we get to private ownership of shares. So what rich people will do with their money is not really a big problem.

As to what happens to 1 when the stock market crashes is simple : the money simply disappears. The money only exists because A lent to B, B lent to C, and C lent to A, after which A, B and C each claim to "have" the money that was lent. When the chain collapses because of stock market tanking or bankruptcies (or both), there is no more claim, and so that money is simply gone.

What happens to 2 is that the government interferes. This has happened before and will happen again, as they say. The government will inflate it away by printing money. This allows them to "be generous". They compensate some of the loss, generously. Say they crash the dollar by 50%, then, in one year, give 25% of that printed money to help out with pensions. How generous ! Somehow this generosity means that the receivers have to move to a smaller apartment, but hey ... look at all the money they gave ! Needless to say, 75% will go to the at that point in time favored rich to "compensate" them for the effects of 1) and the money printing.

THIS. People need to understand this, as it is a snowball effect. Bush Left a 5 Trillion dollar debt (a 500% increase in debt). Then he signed TARP, and handed the mess to Obama, who distributed the subsequent rounds of 'quantitative easing' (I say this to ensure that neither politician gets a pass). We now have $20 Trillion debt, a 400% increase in the 'money supply', along with 0% interest rates for YEARS, and the economy still sucks. In some sewing circles, that's called 'deflation'.

A lot of (the initial, e.g. Belgium / France / etc) EU countries haven't even figured out how to deal with a potential increase of interest rates, let alone pensions. And even with declining interest expenses, they're running (ever increasing) structural deficits.

The only advantage that the US has over the rest of the world is that generally the dollar is still seen as the global reserve currency, meaning demand will at least be there for various reasons.

I've generally not been a big believer in cryptocurrencies, but perhaps Bitcoin could be a decent store of value if the world stays hooked on things like QE and the likes.

On the plus side interest rates going up will actually resolve the pensions crisis...

I am comforted seeing articles like this calling for a top; which make it less likely to happen.