Using a native toolkit built for Rust has massive productivity benefits when writing programs in Rust. Last I checked the GTK bindings for Rust were rough at best, I don't even know if they exist for QT.
The people using the DE don't care what programming language it's written in, so it's absurd to ask users to accept a rough/inferior/buggy/inaccessible system because an experimental UI library was more enjoyable for the developers.
They won't have to accept anything if the developer never writes code in the first place... this is a ridiculous argument. How do you think most open source desktop software gets made?
