An actual quote rather than you just saying such would be more useful. The internet is full of things people repeat that aren't necessarily true.
Hm, you didn't really though?
Your numbers show an increasing percentage following the Mateus drama along with the rules implemented limit ownership of multiple homes to increase availability as the existing design wasn't working.
The trend would be furthered if the Firmament opens as a full district, (today it would map to 69% using your own numbers). The only real downward movement of the percentages is linked to massive population growth with 4.0/4.1.
Also, there's the other issue with the census site that active is defined by completing the 5.0 storyline and so could include people who haven't played since close to the launch of 5.0 leaving it as a number that can't go down even if half the population of the game unsubbed. It's not unreasonable to suspect the numbers used are inflated which would lead to the trend in availability percentages being higher.
The comments around JP servers more indicate there's a technical limitation, if there wasn't and the devs just wanted to maintain a certain percentage of housing availability they'd add wards where needed even just on a regional level.
The only source I'm seeing is census which, while the only baseline, is flawed in its current implementation and doesn't actually support your stance even with that flaw likely causing inflated numbers?
Also...
You've skipped over the 5.1 increase and your note on the recent increase is wrong from what I understand. The translation I have is that they were going to add even more but covid is impairing the ability to do so, and the demolition timer note is that it was going to remain stopped.