[geofft]

Isn't most of the housing stock that is used as an investment property occupied, i.e., being rented out?

Also, the flip side of the argument is that not building lots of housing doesn't necessarily mean that people aren't going to move there. If you have a bunch of housing that is "not for existing Bostonians" as the page says, and it's getting rented/sold, one common reason is that your city is an attractive employment location for high-paying jobs (for Boston I'd guess tech and biotech) and so people are moving in. Not building new housing for those people doesn't immediately make your city unattractive; it just means that this crowd of people with high-paying jobs will take over your existing housing stock because they can outbid "existing Bostonians."