Eh. A lot of hyperbole on all sides here.
All games find a floor... But Civ 7 has found a floor which is lower than Civ6's on absolute terms, and about twice as bad on relative terms. Unless the way players consume both games is dramatically different we can say that Civ7 is doing a lot worse than Civ6. And that's not good news.
At the same time that doesn't mean the game is about to die. It has found it's floor, so hopefully that floor is enough that it can go back up. It honestly could be the case that the floor is high enough that Firaxis can support it for a full lifecycle - to give the worst example possible, Everquest is still alive... Though I doubt Firaxis would be happy with that.
Speculatively, I would argue that since incremental changes in recent patches and the DLC have only marginally shifted player counts, that something drastic is probably needed. And the recent changes are undermining ages by promoting snowballing... So... I think there's hope if Firaxis follow the inevitable?