Vacancy tax would be great but it's way harder to track vacancy than ownership. And it wouldn't help with the rent. Money would still flow through this channel from people who get their income mainly from work to those who get their income mainly from capital. And there are so many channels through which this process happens that if we could limit this one a bit it would be something.

Hoarding limited resorce that everybody needs is one of the societally worst things somebody can do with money. We'd be better off if that money was redirected to literally almost anything else.