A credit system actually already exists. It’s the Grand Company Seals, in form of your Grand Company rank. Second Lieutenant is required to purchase any housing. It fits the lore and it is achievable through both combat and non-combat means.

Problem with anything sold by NPC is that it does not scale well as the game expands. What used to require full raid can be solo’d an expansion or two later. What was an astronomical sum to most is now gained by simply going through MSQ. And once you have set a rate, it would cause tremendous backlash to raise that rate. Existing players would get upset that what seemed to be almost within their reach a while ago is now pushed further away. New players would feel that they can never catch up so they lose the will to continue.

If SE actually considers to make housing availability to be tied to special achievements, it’ll need to be implemented in a new residential area to avoid backlash. So it would not solve the issue with existing housing areas, and controversy is likely to ensue among the “top” players (whatever the type) on where the cut-off line should have been.

And if an exclusive area is set up to top-raiders, drama would ensue among FCs and static’s on who gets into the raid first once it becomes farm status (or even before that). And a large sum of gil (or real world money) would likely be given to the top raiding groups to “escort” their client to get the achievement needed for housing.

It’s a scenario that benefit only the top 1%. It’ll do nothing to decrease the demand.