Also incumbent landlords who are sitting on paid off properties. A landlord with an old building has less vacancies if he doesn't have to compete with new construction.

That doesn’t make sense to me. If a landlord wanted the advantages of low vacancies due to low rent, why not just lower the rent?

They also need to maintain resale value when they decide to let the property go. These are investment vehicles first. Homes second.

I'm not sure it works that way. I'm no landlord, but property tax is not subject to rent control reductions as far as I know. The primary beneficiaries of rent control are the people who are the renters, and are often able to extract value out of it from buyouts in the tens of thousands of dollars to as much as hundreds of thousands of dollars.

In a city like SF, vacancies is not a thing. Actually for that matter, in any market the market price is some equilibrium price that gives a certain vacancy rate.

Much if its value is in protecting existing communities who would be displaced by rising rents, so yeah, it favours incumbents.

I think existing communities is interchangeable with incumbent residents. There are probably better ways to protect mass displacement of low-income residents since rent control only applies to certain really old buildings. Preserving the community in itself is not a bad thing to do but a lot of these policies have also been blocking the building of higher density housing, which could satisfy everyone (keep old residents AND let in new residents).

...who are low/fixed income at the time.

"Affordable housing" is a completely separate issue.