> It doesn't really matter if you add new stock at the top end or at the bottom end, people from the bottom move up, and make space at the bottom for new entrants.
Only if the old housing stock isn't torn down to make room for the new housing stock. Thankfully property taxes aren't driven in the potential value of land (rather, they are tied to actual value), but most small scale landlords of affordable housing right in it are in it for property appreciation, not rents. Their exit plans are often to sell to a developer as tear downs.
> This means they will only build the most profitable units, the high end today.
There are costs are too high (to acquire property and build) to justify anything but going after the highest end of the market. Once the high end is say satisfied, money (investment) could simply move away from housing if it can't make any money there. Money chases the highest returns between all of the markets, not just one market.