> Rent control directly (or undirectly, depending on your views) decreases the supply of rentable units
But it doesn't. Rent control never takes away any supply. Even if rent control were fully abolished, that same person would still need the same amount of housing (presumably even in the same unit). From a "supply" perspective, it's literally a 1 to 1 -- no change in unit counts, up or down, in any way.
Rent control has no influence over "supply" (unless you are secretly hoping to force a lot of humans into homelessness or something). Rent control only determines how much a person is forced to pay to keep their own housing (housing already built, and that they already occupy) consistent.
> Your seatbelt analogy implies that anybody, at any time, can obtain rent control.
Correct, and that's absolutely true. Generally speaking, legislators could pass a law today to give everybody and anybody rent control (at either the municipal / county / state or federal level). It's literally just a pen stroke away.
There was a time when seatbelts weren't law, and it was harder to obtain them. We recognized how unequal of a situation that would make things, and thus mandated through regulations via law that all cars have them. (Just as right now, it's really unequal for some people to have rent control, but others don't).
Generally, every argument against Rent Control, is really an argument about how it sucks that they don't get rent control themselves. The problem is not rent control, the problem is a lack of enough rent control. If rent control was universal (applied to 100% of rental units) then most of the perceived problems with it would vanish overnight.
> If I take 10% of the seats and allow people to reserve them for personal use indefinitely
That's not an better analogy, it's a unfair one. Rent control does not give people the ability to reserve lots of housing units indefinitely, and rent control doesn't actually distribute or redistribute anything. It lets households pay for one (1) unit at real-world rates (which is exactly the same amount of housing they'd still need and use without any rent control, except that household would suddenly have to give a bunch of extra money away for literally no reason).