I don't know what else to tell you. If you follow what is happening in GTK4 you will see that they are moving things out of GTK. This is what they are doing at this current point. Also I hope you understand that this stuff is all open source and nobody is "taking over" any projects, it's simply whoever shows up gets to make the decision. It was mostly GNOME that was contributing upstream during GTK3 so that's what happened.
"How is it not clear, i explicitly provided Win32 as an example right after the part you cut at."
Right, if you want Win32 API, you would use Wine for that. But if that's not adequate then please explain. And yes I know there are bugs and missing features in Wine, so that would be the place to start instead of trying to make a new toolkit that would also be incompatible. If you cared about compatibility with old apps then I'm actually very confused as to why you would be asking for a new toolkit, old apps that don't get updated would never use it.
"...Win32 does get new features and bug fixes all the time - even the pre-XP controls (i mean the non-themed common controls) still get updates."
Okay but which updates. If you want something implemented in Wine or you want something backported to GTK 1 or Qt 2 or whatever then you need to mention that. I can't guess what it is you want.
"So you have to bundle the libraries with your application which not only bloat the application itself but also version lock the dependencies to whatever you bundled, including its features and bugs."
This is an entirely separate problem of which there are multiple solutions to. Let's not get into this though, take it one step at a time.
"it wont get a Wayland backend because the bundled version you have was never updated for it. It also wont get high DPI support for the same reasons"
Ok these are real things to ask for, that someone could feasibly attempt to do. I would say just skip all the other things you asked for, cut out the rest of your comment, and just ask for this, because these are real missing features that could be put into an action item. But actually, I don't think either of these would be technically possible for a lot of apps. If an older app uses Xlib functions then that won't work with a native Wayland backend. If it draws pixels to a buffer and doesn't calculate scale then it won't work with high DPI. (You would be surprised how many older apps actually do this...) So that is unfortunately not something that can be fixed in all cases by trying to backport things to an old API and I think you would largely be wasting your time trying to implement those things. At some point you have to accept that the old APIs are just not suitable for some things.
OpenGL is not really a good example because while it might be easy to add some things like vsync and triple buffering, other things (like GPU parallelization) are basically impossible (or extremely impractical) to try to hack into in the GL implementation. The fixed function mode is always going to have its limitations.
"What i want is simple: a stable ABI so that binary applications can keep working in future (think timeframes of 20, 30 years and more, not just a couple of years or whatever)"
I mean, already I mentioned how it would maybe be possible to get this in some limited form. But someone has to actually put in the work to get it. Keep in mind, it is not helpful to keep requesting that small open source projects do things exactly how a trillion dollar corporation is able to. You are wasting your time asking for that.