It's not sustainable indefinitely but most large landlords have significant capital reserves as well as revenue from a diverse portfolio of other properties. So they can afford to take losses on vacant properties for a few years while waiting for the market to recover.

This is to the detriment of reality, to the actual structure of the city, therefore it must be curtailed.

Vacancy taxes?

You don't make loan payments; you make interest-only payments and are speculating on future appreciation on the value of the land, a proxy for the local economy.

It's not sustainable indefinitely, but it can go on for some time. And if the economy picks up, then all is right again.

You are speculating with other people’s money and at very high leverage. If the market keeps going up the amount of profit can be spectacular and the losses while also potentially spectacular may not be so bad if you don’t have any collateral in the first place.