That blogpost you cited says that there is a 5% vacancy rate. That's well within the normal range. So I don't see how it supports your argument.

https://www.reddit.com/r/badeconomics/comments/musne8/dispro...

Fair— I kind of picked one at random. Obviously part of the point of that dude's effort is that this is a difficult thing to get numbers on. Here's some other coverage around responses to a planned vacant unit tax:

https://www.blogto.com/real-estate-toronto/2021/02/real-esta...

Raise capital gains taxes on land. If they don't sell or rent it's their loss.

If you build more prices will come down and these "investments" will become nonviable.

Maybe, but that is difficult when a) the "investors" are split between connected, powerful people who really don't want to see their investments go negative; and families just getting started who could be utterly ruined by an underwater mortgage, and b) the fact that the housing situation is a crisis now; there isn't time to faff around and hope that ten to twenty years the invisible hand of the market does this or that.