Xlibre

So... [random X11 application] isn't specialized, complex, with a ton of dependencies? I mean, if you're trying to run an X11 application remotely, you're going to need at minimum full client xlib+whatever else on one side, and an running xserver+whatever else on the other side. In other words, by definition, both sides need to have everything necessary to run said application, and *all of it* is outside the scope of what ssh provides (ie a forwarded TCP connection)

Along that line, nothing prevents [Portable] OpenSSH adding support for wrapping waypipe, eg by adding -W and associated configuration options. There's plenty of precedence, not just with X11 (via -X) and various authentication agents but also things like scp/sftp which work by forking off separate executables (on both sides) and shuffling data between the two.

(I would also point out that openssh's -X option claims to support interacting with X11 "Security extensions" by default; I don't know exactly what that entails under the hood but it's clearly more than just setting up a port forward and setting $DISPLAY on the remote side)