The problem is that to make it flexible in a way that allows for GUI construction blocks, with Rust one would end up with Rc<RefCell<>> everywhere, like it happens in Gtk-rs.
Is that all that bad of a thing? You could combine them into a single type, which then becomes a type marker for "is / related to a UI element" and perhaps more palatable to type.
Plenty, decrease in productivity, basically back to Objective-C before GC/ARC, or doing COM in C, lack of support in GUI designers that need to always map to the same types, and building on top of that a big attrition for component libraries.
Not sure if I'm missing something, but how does Rc<RefCell<...>> equate to "objective-c before gc/arc"? It seems roughly equivalent, in the same way that smart pointers are roughly equivalent: the lifetime is inferred by use, and retaining cycles are a problem. Arc could be a little bit more efficient (which does matter in some ui work), but I don't see how it'd change the use of a framework, and iOS and OSX devs have used objc with arc for years successfully and often happily.
