Sorry, but I think you're mixing up personal houses with FC houses. Personal houses are restricted to one per world per service account. The only people with more than one personal house were grandfathered in, two years ago. It's now impossible for people to get more than one personal house and that number of grandfathered housing is only going down as people stop playing (auto demolition) or have to demolish themselves to move worlds.
However, any service account can still have up to 8 FC houses on the same world. They'd have to create 8 separate FCs and level them all to rank 6 and have four members and finally the money to buy the houses, but there's no actual restriction on the number of houses other than the limit of characters. Two years ago Square Enix changed it so both personal and FC housing were restricted to 1 per world per service account, but they removed the FC restriction only a week later. They've never said why, but I think it was probably due to issues with passing FC leadership with an inactive leader (or just wanted to temporarily pass leadership to someone else). I know that the housing mini website says FC houses are restricted, but for whatever reason they've never updated that part of the site.
Also, when you buy a house in-game there's no mention at all of housing restrictions (except when it won't let you buy it) and there's certainly no link to the housing mini site (the only place that actually say about the limit of houses). So it's more than possible for people to buy houses without even seeing this mini site or knowing that at one time SE placed a restriction on FC housing.
Basically my point is you want to remove multiple housing from people, but as long as the game lets people do this, they're not doing anything wrong. Greedy? Yes possibly, but they're not breaking any game rules either so it's unfair to suddenly remove their houses. They also had to fight to get those houses in the first place, using the same systems as everyone else, they're not just automatically handed a house. It'd also impossible to compensate for loss of property too. Yes could refund the base house price, but it's unlikely they'd have anywhere to store all the furniture without a house and they can't really compensate for time/effort/Gil in decorating them either.
Much easier for Square Enix to just work on adding new houses.