Anyone else worried that the increasing amount of big companies, institutions, sovereign funds, ETFs, rich countries, and even Gulf states etc... all amassing ungodly amounts of BTC, could drain OTC trading, and could start to siphon the available supply

Many of these entities make their purchases OTC (over the counter), so it usually doesn't affect exchanges directly too much.

But OTC doesn't mean unlimited supply. Nor is it in a total vacuum.

Those coins have to end up on those OTC trader hands somehow.

The problem is a lot of these new whales and mega funds are hoarding BTC and not planning on selling any time soon.

So that OTC money is moving increasingly more in one direction, and not a lot is gonna come back to be sold.

What happens if these growing mega funds want to load up on more coins, but there isn't an OTC seller available, or the sellers don't have enough coins?

They could increase the price, but if they increase it too much, it's gonna get buyers to go on exchanges.

OTC traders are gonna have to fill some of those gaps somehow, and the next option is with coins from exchanges.

The actual Bitcoins available on exchanges for people to buy, had been dropping since last November:

Declining available supply on exchanges vs price