But that's a phony example. When talking about rental accommodation, you're talking as if it's a fixed thing, with a fixed amount of tenants, a fixed amount of landlords, and a stable situation.
By your same argument, no worker should ever charge an employer more in wages than that absolute minimum that worker needs to survive, plus a small percentage, regardless of how in-demand your labor is.
Sure, an existing landlord shouldn't raise the rent on an existing tenant, so that's why San Franscisco has laws preventing that. But the net effect is that the private sector doesn't invest in housing, leading to outright shortages. That's the same thing going on with the toilet paper at the moment. Toilet paper is "cheap" right now, but you just can't get any, and in San Francisco the rent-controlled apartments are cheap, but there are less cheap apartments than ever, so you just can't get one and the people who have one are like the toilet-paper hoarders.
Also, you state that if there's less demand, rents go up. Which is just completely silly. That never happens, which kind of clearly points out that your understanding of how the rental market actually works must be wrong. If there's less demand property prices start falling, and they sell up. They don't raise rents, because that will only cause more people to move out, in a vicious cycle. In low-demand situations, you lower prices/rent until you hit equilibrium. If the equilibrium point is below your costs, then you go out of business, you can't magically "raise prices" to fix it. In housing, that means a sale of the property and it likely getting torn down to build a different type of building or accommodation that's in higher demand.
The point from San Fransicso is that they were using your basic logic when they brought in the rent control laws, and it actually caused about 30% of that type of dwelling to get demolished and turned into luxury condos instead. Overall, the net effect was that there were less rental properties and 7% higher rents city-wide.
Sure, you made the point that someone else could spend their own money to make low-cost housing and rent it out at low-cost, and technically make their money back. But that fails to account for the money they lose by choosing to do that over better-paying investments. If you're saving your money at 10% interest and I tell you to give it to me and I'll pay you only 3% interest, but it'll save me on rent, why should you do that?