Who's selling the 'new' rooms developer or other people who originally bought in?
It'd only be concerning if the developer still hold a lot of unsold rooms.
If the people paying the mortgages, and ultimately once they got repossessed, the bank, owns the rooms, not the developers, the condo fees will still be paid so it'll keep running
If the developers still hold a lot of room, that also give them a lot of power in the condo meeting to vote things their way too
If the condo were sold to the Chinese back during the boom years, it's understandable that it'll be empty now, but there's also issue with some creative mortgaging by thais getting mortgage for way over value of the room to take home the cash difference and sell on to the Chinese, that could blow up if the Chinese owner decided to be delinquent since they haven't been back in years