It... really isn't. Like you said, remote X was barely usable even over an entirely local network. Most applications these days are also not designed for it, using loads of bitmap graphics instead of efficient, low-level primitives. So you end up being just one tiny step away from simply streaming a video of your windows. We have better tools for doing things remotely these days, there's a reason approximately no one has used remote X after the mid-90s. It's a neat party trick, but I don't blame the Wayland authors for not wanting to support it.
In the 80s/90s this wasn't feasible due to network latency and bandwidth, but it's pretty common now to do exactly this, with VNC and other remote desktop protocols.
In the time when wayland was invented it made sense because we did everything purely local. But now it's as outdated as X11 was in 2010.
And yes I still use it a lot. It works well. Networks have become a lot better and even most cloud compute I use is geographically nearby.
What made it slow back then was that I only had a 128kbit uplink at home. And the uni had 2 mbit for the whole computer science building :)
People complained of no forwarding in Wayland when it was invented.
For example, the remote mail client usecase I was replying to is simply done with a webmail client today.
But this doesn't work on your phone, or on a Windows or macOS device, right? That's what I meant by flexible, X forwarding fits a pretty narrow set of usecases, while on the other hand keeping programs on the clients and data centrally located on a server allows for a whole lot more options for how to interface with that data.
(To be clear, nothing wrong with X forwarding! It's a cool tech and I'm glad you have a use for it! I'm just arguing that it's fine for Wayland to not try to support that kind of thing, because we've got other ways of working remotely now.)
There is not a web tool for every use. And web tools are not better for every use.
The main advantage of x forwarding for me was when I'd randomly need it and had nothing set up ahead of time. Hopefully it starts getting installed in distros by default eventually.