How does that cash end up in somebody’s rent check? Presumably the people selling to the oligarchs are not renters?

If the cash was able to get to renters via some kind of trickle-down effect, we’d all be happy with the resulting economic prosperity and then the resulting rent increases would unfortunately capture some of those gains. Increases in rent caused by increases in income aren’t going to exceed the increases in income, that violates causality. It’s only a problem if rent goes up first and renter incomes don’t increase to make up for it.

There's no "trickle-down" effect. It's a trickle-up effect in terms of (1) rents necessarily raising in reaction to the dwindling supply and (2) the ensuing speculation bubble in "hot" neighborhoods.

Both of which royally screw over long-term tenants living on flat incomes -- i.e. "the middle class".

What do you mean by dwindling supply, we are talking about investors buying properties and renting them out. That doesn’t decrease supply in the rental market.

The thing is - quite often they aren't renting them out; but simply holding them as "assets". Or when they do rent them out - they tend to raise the rent. Which is why the supply of affordable apartments tends to "dwindle".