From a strictly ethics point of view, government in the general sense exists for its people and their well-being. I think this should be a fairly uncontroversial starting point.
Now the question is whether one can show that the people are in a very general sense better off if a desirable city is affordable accessible compared to the alternative. I say “very general sense” because I want to avoid any argument that involves the minutiae of utilitarianism; it should be overwhelmingly in favor regardless of your exact ethical framework.
With that in mind, my argument would be that [cities should be affordable] is the obvious ethical choice if and only if the city in question is designed / planned in a pro-social manner. Where pro-social is defined such that the addition of new people is positive.
This should sound fairly tautological so let me give a few examples:
Pro-social design:
shops that rely on foot traffic are pro-social because adding more people (housing etc) to the immediate vicinity increases foot traffic
Mass transportation is pro-social because as more people are added to a particular neighborhood, this justifies expansion of train lines, bus routes, etc
Anti-social design:
Shops that rely on car traffic are usually anti-social because (a) adding people to other neighborhoods can still increase traffic to the shop, (b) removing parking space to add housing is a net negative to the shop (as parking capacity dictates shopping throughout)
Road systems are anti-social because adding more people to the system increases traffic at a nonlinear rate (ie: a road system at capacity decreases in throughput as more cars are added)
In other words, cities designed like cities benefit from more people, and thus everyone benefits from lower housing prices.
The kicker is, the United States doesn’t have a single city designed as a city. We have dense pro-social urban cores surrounded by anti-social suburbs and exurbs.
In contrast to a place like Tokyo where literally every resident has a bus or train (or combination thereof) to get downtown, America has split between two designs, and progressing along one design hurts anyone living in the other.
A noteworthy corollary is that if you live in a suburb or exurb (and are a homeowner protected from rent increases), property values going up to the moon are almost always positive, because your environment is anti-social in the first place. While in the city core it’s more complicated, because your property value going up raises your net worth but harms the pro-social environment around you.