The problem with making personal houses instanced is that you remove at least half of their appeal. It would be like making it so only you could see your own glamours.

Let's be honest, if you unsubbed before HW and haven't returned for an expansion there's a 98% chance you won't be back ever. If they reclaimed all the personal houses of those people, and all the FC houses where no member has been subbed since the expansion, that would be the equivalent of adding more than 2 full wards in each of the 3 housing zones on my server.