I think the trick here is to not co-mingle the development of UI toolkits and the software built with them. Agile works fine if the UI framework itself is its own project with its own lifecycle, suitably long sprints, etc.

This goes against the pattern that's most often observed in the past decade of every product hand-rolling its UI from scratch, though.

I've worked at companies where to save a few thousands € to not buy a Qt license, they'd basically pay 50x more to have a bad copy of it that was also much slower due to using electron.