Affordability laws decrease the incentive for improvement to the physical structures being invested in by developers
Affordability laws bring in tenants who have less to offer the community over all, including money (to spend on other things like retail and improvements to their own homes), educational attainment (to help school rankings), or crime (the poor commit more violent crimes than the wealthy)
People use their homes like bank accounts; if property prices cannot be depended to go up, then most of the net worth of Americans will be stuck in a slow-growth asset, thus hurting the wealth accumulating ability of normal people.
Prices operate a signals for desirability; if these signals are distorted, then people who have lost the bidding war to be in desirable locations will nevertheless get to stay in those desirable geographical locations because of government fiat.
The poor will not benefit because the wealthy adapt faster than the poor; they build different communities, they make donations to pass legislation, they do things to avoid paying taxes that fund the projects, passing those costs onto those who are supposed to benefit from them. The last time a massive affordable housing experiment was tried, white flight occurred, and the inner cities turned into ghettos.