It currently requires level 50 and being a 2nd Lieutenant in your Grand Company before you can buy a personal house, and an FC rank of 6 for FC houses. I think if anything the requirements are a bit steep already (and the prices even more so). Both requirements and prices were designed to limit the number of players who would try to buy houses, because SE wasn't providing enough of them. They need to take care of the deplorable limits on space, so that they can safely allow everyone to get houses, and then drop the requirements and prices down to something more reasonable.
Agreed. They need a new model that allows them to dramatically increase the space, not just settle for adding a ward or two every 6 months to a year and act like that will satisfy the demand. They need to redesign the system so there will be enough houses for everybody, even on the high-population servers.
Instancing the personal houses is the most obvious way to remove the limit on space, but not the only one. Take LOTRO as an example: It has a similar housing system to FFXIV, with a few locations (four in their case compared to three here) that each have a collection of wards (or neighborhoods by LOTRO's terminology) where a bunch of people have houses together. Both games have 30 houses per ward or neighborhood. The difference is that LOTRO has up to 250 neighborhoods in each of four areas, whereas FFXIV has 16 wards (well, 8 wards and 8 subdivisions which are essentially the same thing) in each of three areas. That difference between 1,000 neighborhoods in one game and 48 in the other is what makes that shared neighborhood pattern work there and not here. If SE wants to keep using this type of shared housing ward, then they need to actually devote the necessary servers to housing, just like Turbine did for LOTRO. We're stuck with these ridiculous limits on housing for the same reason a lot of other things are lacking in this game, because SE doesn't want to spend anything on their infrastructure.
As to how LOTRO handles the abandoned house issue, they have a maintenance fee of a few silver per week that can be paid ahead up to six months in advance. Past that, if your maintenance fees aren't paid your house is locked so you can't use it, and after a period of time (another month IIRC) a locked house will be foreclosed with all contents going into escrow for the owner to pick up when/if they return. Combined with the fact that houses there are drastically cheaper than here, and there are plenty so you can always get another, not many people are upset with the foreclosure system. (There were, however, lots of complaints about their first attempt at it, because initially the escrow that holds the items from your house was going to be time limited as well. To get a system that players would accept, they had to drop that rule, and keep escrow indefinitely. There were a few other changes as well, like extending the amount of time you could pay ahead to 6 months as it was originally shorter.)