From https://wiki.ubuntu.com/Wayland "The X11 protocol was designed around running graphical apps across the network. While some people use this feature, it's far from common. Wayland drops this requirement as a way to greatly simplify its architecture." X client and server are usually the same machine, but they don't have to be. While on the road, you can use your notebook to open a Gimp session on your home machine…
NoMachine developed a couple of libraries called NX back in the day, which transferred images and image deltas over with high compression. We used this instead via in the same project (with X2Go), and I reimplemented the same stack to my university where 20 something users connected remotely to a single "terminal" server to do remote research, and it worked like a charm.
While I like Remote X, it's still very inefficient even today. So, unless it's made extremely efficient over normal internet, over residential connections, it won't be missed.
Moreover, the rarity of projects using Remote X, or abstracting it with libraries like NoMachine kinda validates the idea is the feature is considered a novelty and not used much.
While I like the feature, I have feeling that it won't be missed or sought after much.
On the other hand what killed X is its code state, rather than the complex architecture. It's haphazard development over the years which made the code unmaintainable.
Addendum: Remote X made sense back in the day. Carrying minimal data, mostly terminal windows between terminals and central mainframe/time sharing system, over relatively short distances. I guess it's never designed and considered for long distances like today's internet, hence it's left to the wayside.