Unity's (at least when I started using it with 14.04) UX, screen space usage and performance was so good. Global or locally integrated menu bar and making it searchable were great decisions. I'd much rather canonical had been successful with desktop & mobile consumer OS and upstart rather than with snappy
My current phone is a Volla 22 with Ubuntu Touch, it's pretty good (although the battery doesn't last me a day, sadly). To be honest, though, I kinda miss the scopes of the OG Ubuntu Touch.
This duopoly isn't going to be broken up because of Rust. Linus recently said that developers aren't taking to it for the kernel.[1] There are serious issues with Rust that bar any real adoption from the overall complexity (async, procedural macros, unsafe ergonomics) to the problematic toolchain (too many dependencies and slow compile times).
Linus the guy who was against debugger and source control. He is dealing with long time C users working on a C project that just recently has allowed Rust, many work for companies that dont use Rust.
Meanwhile Oxide is building a whole datacenter infrastructure in rust including firelmware, network kernel models and store subsystems, hyperwiser userlabnds in or on Illumos. And say almost universially possitive things.
The people who make cosmic also sell laptops with Rust written firmware. Some of them also develop Redox OS, a Rust OS. They in general love it.
But I guess because mass adoption in Linux hasnt happen it will be impossible to create a new popular UI toolkit.
EDIT: Almost forgot about Unity, which has arisen from the ashes.