First of all, that population website is hardly reliable, but you know that.
Secondly, over 10,000 unique guilds have killed heroic Sylvanas, whereas only ~5,000 have killed the equivalent FFIXV encounter which is currently Savage Asphodelos. The latter is definitely more recent, but the barrier to entry for that encounter is, even now, much lower than Heroic Slv. All of this while a decent percentage of WoW content creators are lauding FF as the most alivest and bestest MMO on the planet right now.
https://www.warcraftlogs.com/zone/ra...lty=4&page=220
In fact, if you really start to parse the FFXIVLogs data against the Ironforge.pro data for TBCC, you can make an argument that TBCC on it's own has a raiding population that is just as healthy as FFXIV currently. The breakdown of class parses on FFXIV logs shows that the total number of parses isn't all that much higher than the TBCC numbers.
My point here is, WoW isn't dead, FFXIV isn't dead, and if you're going out of your way to find out ways of saying they are, to the point of cherry picking unreliable data, you should honestly consider just stepping away. I know it must feel like you're vindicated when you don't feel the same way about WoW as you once did and can farm "hate the thing" attention in cesspools like this, but it's not healthy.