The only issue with a simple subscription check is that it doesn't catch any of the players who have bought houses but just keep them around for either reselling or found that they aren't really that interested in housing and leave it empty.

It's a pretty significant portion of the homeowners in the game.. They're not unsubbed, they're just off doing other things and the house gets no love whatsoever.
First, these types of homeowners need a way out without losing their gil, so even implementing something like house trading would help a little bit so these types of homeowners can get their money back AND the homes can go to people who'll take better care of it.

It's not a one step fix all, there are many proactive things SE needs to do, but certainly sitting on it and releasing new wards once every year will not solve it.