Unless the translation I have of the live letter is wrong or was later corrected, you've incorrectly remembered/interpreted dev statements from less than two weeks ago. The same could be true for the quote you're referencing.
That said, I've looked and I'm not finding the interview. If it was a comment from Yoichi Wada he also left before Heavensward released, which is over five years ago, it's also worth noting that the game has also significantly changed since then and we've seen updates to housing to promote availability because the design from back then did not work.
Regarding the comment from Yoshi-P about the price of housing, I did find one linked to the 12th PLL which seems that housing prices were initially ridiculous because they were based on the gil earned by legacy players. So I'm rather uncertain how this is relevant to your point as it really is an example of the devs realising their initial design choices didn't work and changed them, i.e even if the quote you reference exists it doesn't mean they haven't changed their mind since, particularly given all the housing drama in the interim.
This reinforces the question of veracity to me, so until it's proven the comment exists, it's really irrelevant.
For fun... during PLL LIII there was a QA where it was mentioned in response to a question about adding more armour slots that adding more housing has implications around player experience such as server crashes. Which supports the idea that ward limits are a technical limitation rather than design intention.
The translation I have for this blog (post 4.4) http://blog.livedoor.jp/umadori0726/.../54040204.html indicates there was/is a focus on housing as content you can only do in FFXIV (i.e a draw) which, realistically if people are getting frustrated and leaving because of a draw then it's not working as intended. Supporting the view that housing should be accessible enough that people aren't getting frustrated and quitting over it.
There's another here http://ff14net.2chblog.jp/archives/52375894.html where it's apparently noted that S/M/L apartments aren't planned but it's possible once they've solved the plot shortage. Coupled with http://game.watch.impress.co.jp/docs...w/1021088.html where it talks about how apartments are intended to fill the housing needs, that they can be more frequently added, that they resolve issues around chocobo stabling. This seems to also support the view that accessibility is an issue, apartments were created to mitigate that but the focus is on improving plot availability.
These sort of comments lead to my view that the current situation is not what the devs want and they're continuing to make housing more accessible rather than it being intended for only a certain amount of players.
Your conclusions aren't really lining up with your data though. You are correct, there is a linear and consistent image, an increase in the relative availability of housing. But if your conclusion around the 30-50% availability was accurate then there wouldn't have been a need for the latest increase as the existing availability would have still been on the high end (even above it due to census inaccuracy).
By skipping the 5.1 increase you don't show the increase cycle which, when included, appears to now be two increases per expac. This is also relevant to your note on the latest increase as you deemed it outside normal cycles as opposed to what is becoming the norm.
This would have even helped your point ironically as it would have shown a drop in availability rather than increase. Of course, the response to such is, given the current census data is almost certainly over-reported, that drop is more likely due to inaccurate population data.
Further, the latest live letter indicated they're already considering adding even more wards, which again, if your conclusion was accurate wouldn't actually be necessary with the Firmament coming and those resources could have potentially been focused in bringing it in as early as possible.
Also, when Ishgard is releasing is speculation sure but we also know from the last PLL that 5.41 is the last step in the restoration, so it's also plausible that it could come in any patch following that, even as soon as 5.45 or even 5.41 depending on the mechanics of that step.