Why affordable housing should be new?
You are correct, that it would be possible to build new luxury housing and then lower the cost of old luxury housing until it becomes affordable. However, that seems unlikely to actually happen since the owners of the old luxury housing are likely to resist losing money. In some cases, luxury housing may have features (size, density, amenities, etc.) that make is economically unviable to price it as affordable housing.
Reality is that it's too expensive to stay a "luxury" housing, all the labor costs and major renovations just don't make sense from a financial perspective.
The fundamental issue that people miss is that the cost of building a "luxury" housing and "affordable" is not that different. Throwing quartz countertops and a tiny gym with outdated cheap equipment for a few extra k is nothing when the place costs 500k+. Sizes are also not relevant, since newer apartments are smaller than older ones. We can't just build an old affordable house, that's it. We need to start building as much as we can and impact the demand by evaluating the impact of new businesses coming to the area.
You could evict a bunch of people from existing houses and then re-sell those within the constraints of affordable housing parameters, but I doubt that is a feasible method of significantly scaled up expansion the pool of available affordable housing.
See Oakland.
Gradually? That's roughly what's happening in Denver right now - nobody's been building anything but luxury castles for years, but we saw our average rent drop last quarter for the first time since the recession.
Reason? There were 11,000 more units available than there were this time last year and there are 40k more on the way, so apartment managers are having to think about vacancy rates instead of myopically focusing on profit per unit for the first time since... probably the recession again.
https://www.denverpost.com/2023/01/25/apartments-metro-denve...