Ok, I get what you're saying, but this is not different from looking at GNOME 2 (which only ran on X11) and saying they declared a fiat that you have to use X11. Any project gets to pick its dependencies.
No, the parent comment is wrong. There is no "vertical integration" or control. It's just an open source project. If anyone was interested in maintaining the X11 support, it would happen, or at minimum it would happen in a fork, but there is currently nobody.
If someone is worrying about other parts suffering, they should start contributing now. I doubt this will happen though, nobody has turned up over the past several years, which is exactly why they consider dropping it. Honestly I think most developers who even bother with GTK4 are probably fully on board with wayland, the ones who aren't are still trying to use GTK2 and GTK3.
Therefore, as you say, there's no incentive for the GNOME developers to support X11, and the devs are free to say "Use GNOME on wayland, implement an X11 backend for GNOME/GTK yourself, or don't use GNOME" which, in practice, boils down to "Use GNOME on Wayland, or don't use GNOME" and is, effectively, a fiat to use Wayland if you want to continue using newer versions of GNOME.
I also didn't intend this to be judgemental; GNOME is almost certainly the most complete desktop environment for linux. I personally use KDE as my desktop, but most applications I use are not part of KDE (or even built using Qt). Conversely, a typical desktop user could probably get by using just GNOME applications (plus maybe libre-office for opening MS documents that can't be handled within GNOME). IMO a large part of what makes this possible is by being judicious in what is supported.