Before I run this thought experiment, I'd like to know what percentage of top 20-30-50 cities in the world where average income people are not priced out of city centers / downtowns or other desirable areas.
I don't entirely understand your question, but the one bit I do want to address is that it's somewhat tautological that average incomes will be priced out of [the most] desirable places.
The issue with SF that the OP here is trying to extrapolate to US cities as a whole is rather more like "priced out of literally every part of town other than a tent on the street" while at the same time having huge amounts of empty space in buildings, which is a very precarious position and one that's far less common in large US cities/metro areas. (There's also a unique aspect about SF being quite small, city-wise, by US standards and much of its surrounding suburbs also being obscenely expensive across the board; SF itself could suffer big hits to population and desirability and that may or may not have much impact on the rest of the Bay Area).
The issue with SF that the OP here is trying to extrapolate to US cities as a whole is rather more like "priced out of literally every part of town other than a tent on the street" while at the same time having huge amounts of empty space in buildings, which is a very precarious position and one that's far less common in large US cities/metro areas. (There's also a unique aspect about SF being quite small, city-wise, by US standards and much of its surrounding suburbs also being obscenely expensive across the board; SF itself could suffer big hits to population and desirability and that may or may not have much impact on the rest of the Bay Area).