The problem with this argument is that it assumes the problem exists and isn't getting any worse. It is in fact getting worse. The crux of the problem is not necessarily the current steady state imbalance, but the continuous imbalance that results from metropolitan areas adding jobs faster than they can add units of housing.
As long as these high cost of living areas continue adding jobs, particularly high income jobs, you continue to have a growing pool of knowledge workers who can afford these market rates that are unaffordable to the working class pool. Give this pool of workers a big enough carrot, maybe its a fitness center or a pool or some astroturf for the dog, and they will gladly move out of that 50 year old dingbat and into this new construction with the golf simulator in the basement and a garage parkng space, freeing up that 50 year old apartment (which in its time may have even been a luxury building in its own right) for someone who can't afford the above amenities.
To this end its essential to build housing as fast as possible and to create as many units as possible. When you mandate things like x% of units as 'affordable,' often times that leads to developers going back to the drawing board and chopping off entire stories from the build, and fewer units of housing being added to the local area than there would have been had the developer been allowed to build full bore and actually cover costs on a larger build. This ironically hurts the very class of people who would have benefited from those affordable units in the first place.
> Give this pool of workers a big enough carrot, maybe its a fitness center or a pool or some astroturf for the dog, and they will gladly move out of that 50 year old dingbat and into this new construction
This is only partially true though. What you mean is some percentage of people will move into the new construction. But of course, some people will see that the unit they live in is becoming more affordable and decide that they would like to take advantage of relatively lower rent. Over time and on average you are correct - that building nice new homes will relieve pressure on somewhat less nice older ones, but it will take time.
Also, we aren't talking about mandating affordable units, which is a thing in some places, but rather a developer trying to apply for subsidies for building affordable units. I totally agree that we should not put up barriers to building market rate units, but it's also worthwhile to incentivize some artificially low cost units as well to help solve the problem for some people today instead of 10 years from now.
As long as these high cost of living areas continue adding jobs, particularly high income jobs, you continue to have a growing pool of knowledge workers who can afford these market rates that are unaffordable to the working class pool. Give this pool of workers a big enough carrot, maybe its a fitness center or a pool or some astroturf for the dog, and they will gladly move out of that 50 year old dingbat and into this new construction with the golf simulator in the basement and a garage parkng space, freeing up that 50 year old apartment (which in its time may have even been a luxury building in its own right) for someone who can't afford the above amenities.
To this end its essential to build housing as fast as possible and to create as many units as possible. When you mandate things like x% of units as 'affordable,' often times that leads to developers going back to the drawing board and chopping off entire stories from the build, and fewer units of housing being added to the local area than there would have been had the developer been allowed to build full bore and actually cover costs on a larger build. This ironically hurts the very class of people who would have benefited from those affordable units in the first place.