Altogether they control TRILLIONS of wealth.
It's probably why Bitcoin developer Peter Todd has currently been saying that he's open to the idea of an inflationary cryptocurrency.
There are already a huge number of such cryptocurrencies. There are already so many full-fledged blockchains that it is difficult to count them all. If someone needs inflationary cryptocurrencies, then they can either choose from this set or make their own as much as they want.
That's not the point. The point is how Peter Todd's change of opinion from a "limited supply is good" to "inflationary supply is good". We're merely talking about concepts and how theoretically one might be better than the other for the long term sustainability of a cryptocurrency network.
As for trillions, they are not going to buy bitcoins with all these trillions. If I go to a candy store with a thousand dollars of capital, it does not mean that I will spend more than a few dollars there.

It doesn't change the fact that they can put a large percentage of the total supply in their custody, and the question is, "What is the threshold before it becomes truly concerning"?