The libadwaita stuff is also super weird and IMO implemented poorly, and in a way that basically says "if you aren't using GNOME we don't care about you". Want to run an app that uses libadwaita outside of GNOME? Sure, but expect it to look completely out of place since it won't use your desktop theme.
Yes, they were, that is the opposite of what actually happened. The real rationale is explained in the libadwaita release announcement: https://adrienplazas.com/blog/2021/03/31/introducing-libadwa...
>and one of my recent projects simply cannot be built with GTK4 due to features that were removed
Yes, they also deprecated/removed some features that were just broken or not so useful. That's not a GNOME issue.
>the second and third paragraphs still hold
This is just my prediction from looking at some of the existing Rust libraries: There will not be any viable toolkit for general use written in pure Rust. To do so would require a giant investment. If it ever happened, any interested party would want it to be cross-platform because Rust is primarily used as a cross-platform language. And because of that, at best it would become a poor approximation of Qt without any of the benefits. The closest I could ever see conceivably happening would be some WASM-based thing that you can then deploy inside Electron. And I say this as someone who rather dislikes C++ and (in theory) prefers Rust.
>and if GTK5 does eventually drop X11 support, I certainly wouldn't bother with it... unless of course somehow X11 actually disappears.
I don't know why you wouldn't bother with it. The code will probably not ever completely disappear, but X11 itself has already been obsolete for a number of years.
>Want to run an app that uses libadwaita outside of GNOME? Sure, but expect it to look completely out of place since it won't use your desktop theme.
I don't see how this is even a problem. The point of the GNOME app is to follow the GNOME HIG and design patterns. The point of libadwaita is to provide prefab implementations of the GNOME HIG. You can't make an app that follows two design styles at the same time. Like, think about this. Motif or KDE apps also don't look anything like GNOME apps and they look out of place. Windows apps also look completely out of place on GNOME if you run them in Wine, etc. On an open platform nobody can enforce a certain look for any applications. Eventually they will all look weird and out of place somewhere.
I don't know where you heard this, but it's not at all true. This is actually the complete opposite of what they did in GTK4. A lot of the features that were removed were actually GNOME specific features that were simply moved into libadwaita, the thinking was that anyone building a platform on GTK4 can create their own library and put the themes/widgets they want there. Elementary OS also did exactly this with libgranite. The rest of your comment stems from this false assumption, so I think you can retract it.