Because the number of plots in this game can only cater to a minority, might as well make that minority the ones who REALLY want the housing instead of someone who just bought it because it was new and shiny, and hasn't done anything with it for the past 6 months. (These people are still subbed, housing is just not a very big interest to them, but there's no reason to relinquish, so they keep it around without using it, I WAS one of those people, until I gave it away to a friend of mine as a wedding gift)
Houses in the game are ALREADY owned by the rich. Currently, it's a mixture of rich players, some who really care for housing, decorating, etc. and others who couldn't care less but have no reason to relinquish. If upkeep was implemented, it would at least be the rich who really want to own houses.
Every time I pass by houses in my FC ward, and see an empty yard, even an empty plot (owned, but not built) it makes me wonder if these people really deserve to keep their house simply because they could afford it. They don't live in it!
Yes, adding additional wards would have to be an ongoing thing to fix the underlying cause of the issue, but in the meantime, upkeep would let more people play with houses. At the rate we're going, I don't think we'll ever see enough wards added to cater to anything BUT the minority of the playerbase. Even on Jenova, there are about 7 or 8 Mediums left and that's it. Most people are going without houses, but as time passes it's only natural that more of them would want it, 2 months down the road I expect to see some complains about lack of housing on Jenova.. Somebody posted that the last plot on Lamia sold out a few days ago, a day after that there was a post of an FC that wants to buy a plot on Lamia, give those people a few more weeks and the idea of "I want to buy a plot" quickly changes to "There's no plot anywhere!?"
If SE was willing to scrap the housing entirely and overhaul the system where every house is inherently instanced and they released residential districts in the game with rent that worked in a trailer park like manner, that is what I would imagine to be the best way to implement housing. I'm not an advocate of the current housing system as is, I just don't see any realistic solution other than a redo, which I don't think SE is willing to consider.