First of all; let me be clear; there will NEVER be enough houses available to support all players and this IS intentional. It's actually genius if you think about it.
Housing availability and demolition are one of many rotating tools Square uses to keep players subbed without lapsing subs over in game "lulls"...
If there were a surplus of houses available, players with houses would no longer fear demolition, and would feel free to unsub when they got bored with the game or were waiting for new dungeon/raid/savage/story whatever content... demolition is one of the tools they use to keep you subbed for that month or two when you really have nothing to do (like, right now) and they don't have any new content to give you... that tool would be useless if you knew you could always come back and buy a new house.
Why do they ever turn off the demolition timer? They do it to gain player trust, and they do it in times when they know they are least likely to lose subs, and they turn it back on when they know they are most likely to lose subs.
Why do you think Kugane Housing wasn't available with the launch of stormblood? Because the new expansion is their strongest tool to use to renew / reactivate / gain new subs and keep people subbed for at least the beginning of the expansion... they didn't want people racing through the fresh content just to get to the housing area, so they made you wait until the freshness of the new content was getting stale before releasing their next sub sustaining tool; fresh housing plots... but they also wanted any NEW players to get to that point in the game and get into the housing market... this allowed new subbers without houses to get them and locked them into the "don't unsub or you'll lose your house" group, but kept the housing availability low enough that people with houses MUST stay subbed or fear losing access to that content indefinitely if they ever decided to return.
Basically, if the houses are available equal 60% of the playerbase, they will only release new plots when the active player base has grown enough so that the new plots available combined with the old will still only equal 60% of the active playerbase. When they predict that even with the threat of demolition they will lose a few subs due to lack of new content, like after the first few weeks of a new patch when quick players have already geared their mains and beat most content, they deactivate the demolition timer and let those unsubbers take a break confident they may return with new content. But when they predict many players will unsub during extreme lulls, like at the end of an expansion or patch, they activate the demolition timer to keep them for an extra month or two... but it would be stupid of square to provide enough houses for everyone because then demolition would never be a reason against unsubbing; you can just buy another one easily if you chose to return....