Well I understand your concern but the actual "prime" real estate is in fact virtual...Now i know it sounds like I am making light of you but I'm not. It being virtual allow for things that real world mechanics don't apply to. Supply and demand for "land" won't matter because if you buy a Mansion the system will allow it placement automatically just like a key item automatically gave you the ability to activate the special drops in AV or CC so the only plots will always be there.
Now if you are specifically referring to neighbors and or specific neighborhoods then yes I understand where your coming from and that would be a problem if your friend bought house 1 month before you so their instance is filled. But really the first come first served mentality is the fairest result here and would cause the least issues.
But if you talking about just your plot then land can be expanded virtually to allow you to fill in your plot because the instance already would have been planned and allow for the biggest allowance possible for every player in the instance. Now if the system is made under the lego formation that would infact place a limit on mansions if the instance is at mansion limit which is against the primary design philosophy where one character can do everything so either they would force a move to new neighborhood to accommodate the size of the house or move existing static structures which isn't fair or logical. So I am assuming that the designers are aware of that and choosing the first system.
The last possible alienation you would see would be if your house is the only small house on the block...which really is something that doesn't specifically effect you as a player. It would be an outward expression of how you didn't spend your gil but at the same time is another treadmill that you can jump on when ever you feel ready too since gil/craft level/achievements can be obtained at your own pace. Now things like events like Christmas/Cherry Blossom/Halloween where it is a one time thing until the event occurs if they have outward decorations that would be one instance but even then not somethign we aren't experiencing already with achievements.