My guess would be that talking about ethics when it comes to virtual houses feels a bit blown out of proportion... I've taken a few ethic classes at uni and would have gotten really weird looks if I'd wanted to discuss this. Not because my prof wouldnt have understood video games but because "...are you sure this is an issue you want to discuss in an ethics-class?"
Correct me if I'm wrong but your stance seem to be that no one should own more than one house, regardless of circumstances. You're basically asking people to be holier than holy. (I'm excluding houseflippers.)
You seem to forget yourself that this a video game. A place people come to, to (partially) escape real life for a bit. Not a place to ask yourself "Is it morally wrong if I buy this open plot that no one else seems to want because I enjoy decoarting?"
Are you assuming that people owning multiple houses doing this with the ill intend of withholding them from other people (again: houseflippers excluded)?
I dont believe any of those people have those ill intends - and I dont require them to be saints either, what you seem to do.
First come, first serve - no need to get all high and mighty and ethical over a bunch of virtual houses.
Because the only logical conclusion I can draw here is that, ethically speaking, personal housing should have never been a thing and it should have been FC-only all the way.
It isnt - and now everyone has theoretically speaking the same chance to purchase a house. And if someone goes through the trouble of leveling up multiple characters and earning the gil, they're very much entitled to buy a second house as far as I'm concerned. Because they've earned it. And if someone else didnt went through all that trouble, checked for open plots etc. they didnt earn it. Simple as that.
Everyone has an equal chance here - but equal chance doesnt mean equal results.