What do you think is missing at the moment? Ubuntu has a simple setup page, so you don't need extra software https://wiki.ubuntu.com/MultiseatTeam/Instructions
There's also a number of companies selling desktop terminals you can connect to anything running NX. (Or some other server)
Why bother with multi-seat when you can get a decent SoC computer like a Raspberry Pi Zero W for $5? If multi-seat is an option for you, you probably aren't doing anything highly CPU or GPU intensive anyway.
It's not taking forever. It already happened years ago and was effectively abandoned. Once upon time, back before the X11 server became so dependent on kernel facilities, it was relatively easy to setup a multi-seat box.
And don't forget--X11 is a networked protocol. It's still easy to run X apps from a server to be displayed on a local dumb client. But much like the evolution of the X server itself, the GUI and toolkit projects (Gnome, KDE, Wayland/Weston, etc) have neither the time nor inclination to invest in this use case. And so there's been a slow, two decade-long regression in this regard. And it's only going to get worse.