Where do I start. First off housing has been balanced around populated servers. They've always tried to maintain 1/3rd of server population having access to housing. On smaller servers this at times was closer to 50%.
Secondly, you're comparing apples to oranges. Comparing ff14 housing which is a PVP mechanic to instanced housing in other games which closer equate to PVE mechanics is moot. It's like comparing an Age of Empire solo campaign to a starcraft II Platinum+ league. They're both RTS but that's where the comparison ends. Anyone can finish the solo campaign, but not everyone makes it above platinum. This is also why it's really pointless to bring instanced housing up in the first place. This might shock many but this has been suggested for as long as housing has been around and surprise surprise, it isn't even on the table.
To further insist. You try to make a point of using ressources to benefit a few. Now 1/3rd population is not a few by any measure but, regardless, my counter argument is why do they even make savage and ultimate fights that benefit even fewer? Hint: there's a reason why you would do this and if you're at all interested in game design, please look up cognitive flow.
The current system isn't a mistake. It may have some technical limitations and it's fine to discuss those in the proper context but the design choices the devs made are not wrong they are just different. Again apples, and oranges. Barring the placard spamming that is horrible, the mechanic is difficult to get into but still rewarding when you make it. This is, almost by definition, a successful mechanic.
It's also not that hard to get into housing even on populated servers as long as you dedicate yourself to it.
There are countless people who've gotten multiple houses, lost their houses and got more, upgraded to medium etc.. etc... From my experience they always fit the dedicated player demographic and that's perfectly in thune with ranking high on a PVP leaderboard (which housing is since the more time you have to put towards it the more likely you are to get one).
The only thing that is truly hard to do in housing currently is get a mansion as an individual. And this was brought forward as a request by the community