It is a fallacy to claim that building only luxury housing is a problem. If you don’t satisfy luxury demand, you get massive gentrification as the wealthy push out the working and middle classes. See Brooklyn for example. Years of under building in Boston have led to luxury renovations (Brazilian cherry interiors, Jacuzzis etc) in working class neighborhoods in Cambridge.
There is one number. Housing supply. It’s price is driven by demand. If there’s too much luxury prices will fall and sweeteners will be proffered to attract the middle class. Apartments can be subdivided. Let’s encourage building and not care beyond the number of units. The wisdom of developers when it comes to price point isn’t the state’s concern. Property taxes and the cost of capital alone will force them to rent space below their desired rate.