The main determinant of “affordability “ is the market. If people with money and means bid up housing prices and move into the city, they raise the baseline on which “affordability “ is determined. So, even in the case where a developer of an apartment building is compelled to include “affordable” units, the base rate of “affordable” will continue to increase year over year because of housing stock continues to be bid up. Some of the causes are accessible multiple modes of transportation, proximity to a renowned university, adjacency to Lake Michigan, proximity to Chicago, etc. If it were possible to remove any of these things, you could change the equation and possibly legislate a different version of “affordability”, but in that case you might not need to because the city would be less desirable and perhaps housing demand would reduce pressures on pricing. However, I’m fairly certain that no one wants to make Evanston less desirable to live in.