> Certainly there are still some people who are wealthy enough to keep their vacation homes vacant most of the time (and prefer it that way, since doing short-term rentals tends to involve quite a bit of wear-and-tear on a house and its furnishings). But many list on Airbnb now. I consider this a net positive for society.

You are missing a critical issue. Airbnb makes it way more affordable to have this second home, so now many more only-sorta-wealthy people can afford vacation homes while pricing people out of the local housing market.