You answered your question of why Wayland exists right after asking it. X sucks. Wayland is a very significant improvement, I’m not sure why you think it’s a lateral move.
Also, X works for some cases, but not all, just lime Wayland. Using multiple refresh rates doesnt work well, HDR has no hope of ever working, and fractional scaling is horrible. Wayland has initial support for HDR and great support for the other two.
Isn’t that already true for QT apps on a gtk desktop and GTK apps on a QT desktop?
In my experience, GTK apps feel pretty good on Cosmic, and it has support for automatically theming them with your Cosmic theme. QT apps feel out of place, but this is an issue I’ve had on every non-Plasma desktop, because I struggle to get theming to work.