That said, has there been any fundamentally groundbreaking crossplatform classic UI toolkits released since the 90s? (IMGui is the only one that has seemed interesting but that's specialized and not a general one really)
EDIT: Another commenter suggest only office as example. IIRC, they are building it using chromium as front-end and .NET as backend.
I'm pretty sure that something based on IMGui could be more or less isomorphous to React style rendering, it'd be up to someone to implement it though (and it'd probably be worth it since building applications at scale you win back a lot of time by not fiddling with state manually all over the place).
Using Chromium however is explicitly not where we should want to go (it's basically a kitchen sink in itself), but we go there anyhow (me included often) because it's just so much more quick thanks to progress in dev experience in the webdev area.