I use Flet in a few projects. It's still somewhat young and the declarative move caused a few issues (for me, mainly because I wasn't familiar with the pattern), but generally I'd say it's stable, feature-complete and the maintainer seems to be in it for the long haul. Also it fills a critical gap that nothing else does, binding a decent UI framework and cross-platform capability to a language with an extremely large and diverse ecosystem; that's what I think will really make it endure.