Foreign buyers just don't make up enough of the market, and neither do people with 5+ residences.
Let supply outstrip demand and suddenly they're not a good investment anymore, so people go back to owning them for living in.
Not that this is a good use of a perfectly fine basic housing unit, but it isn't an investment.
Seriously, a "winter escape"? Literally noone needs that. Literally noone should have that until everybody's basic needs are met.
Which won't happen with all these "storage unit" houses there are.
The second home is already slightly tax disadvantaged compared to the primary residence, perhaps it should be "punished" more.
If that third house had some form of tax penalty associated with it being unused/empty they might bother to sell it.
But the housing crisis isn't perpetuated by empty homes in general; empty homes exist because there's an investment aspect of it. If the grandma house was losing at IRS depreciation rates, it'd likely be sold (though a storage unit might cost a similar amount).
With WFH especially now you can work in various parts of the city or move all over the world and at most be tied down to a place for about a year.
Homeowners also never talk about things like paying tens of thousands of dollars when a pipe burst or the roof needs repairing.