Sounds like a hack. Why not just fix the accelerant API instead of spending that time?
This is also just one hurdle of many, anyway.
We have for a long time planned to implement multi-user, and already have it at the kernel/filesystem level. If nothing else, we should run applications as non-root for privilege separation. But this is a separate debate.
Each user will have their own “desktop” with windows, etc. in case 2 users are signed in at once, right? Especially considering app_server does a lot of drawing server-side, it’s not just a “window server” like Wayland, it seems to make much more sense to have an instance of app_server for each user.
This is what Linux does with X11 (and I think also with Wayland.) I’m not sure why you are mentioning Windows as an example here. But we are not either and do not have to copy their designs, if a different one makes more sense one way or another.
There is only one set of functional open-source 3D-acceleration graphics drivers: Linux KMS-DRM ones. Every other open-source operating system, including all (Free/Net/Open/DragonFly) the BSDs, as well as other niche OSes such as Aros or the like, port Linux’s drivers instead of writing their own.
FreeBSD, for a while, had a partial rewrite of these drivers to be more FreeBSD native (even including code formatting and API usage), but eventually they abandoned that and wrote a compatibility layer, too. Google’s Fuchsia/Zircon had their own, but I think these were only for select chips of select vendors, and, well, we are not Google anyway. MorphOS supposedly has one for ancient Radeon cards, but it is closed source.
The fact of the matter is that each of these drivers is massive. The AMD KMS-DRM driver alone, for instance, is over a million lines by itself; and the same is true of Intel, Nouveau, etc. The entirety of Haiku is “only” ~3.5 million lines. We are talking about individual drivers that are nearly as large as the OS itself.
There is no possible way we can replicate these from scratch, on our own, in our spare time, to the level of quality and performance that the paid developers (most of whom work for the companies they are writing drivers for the hardware of!) of the Linux kernel are.