The cartels and PE investors are investing in NIMBYism. It’s a bet that resistance to construction will continue to keep supply artificially constrained, forcing prices up.
If that resistance is removed these investments will underperform.
A vacant property still has to be insured, maintained, heated/cooled, taxes paid, mortgage or commercial loans paid.
So if the supply is high enough that landlords can't afford to hold enough of them vacant to maintain high rents, they will eventually have to rent more of them.
The landlord cartel theory requires landlords both to have unselfish solidarity with other landlords, and be selfishly greedy against renters.
But in rental market, the demand is not dynamic. You HAVE to get an apartment to live in once your lease is up. If 80% of the complexes around you are on this and on top of that the independents also set their pricing based on the market controlled by the majority, then they can charge you whatever they want. You HAVE to buy and you HAVE to get it with the price dictated by them. There is absolutely no supply and demand in this at play.