If you have enough permissions to intercept keystrokes in X via network monitoring then most likely you have enough permissions to intercept keystrokes in wayland e.g. by ptrace.

You're not using TCP for local X, it's all Unix domain sockets and SHM. Hard to get any faster on Linux.

One could certainly implement feature emulation to preserve backward compatibility with evolving hardware. Or just deprecate feature, X over network was pretty much broken for a long time.

Not saying Wayland isn't an improvement over X but a lot of supposed limitations of X are often misrepresented and exaggerated.