We've had ~15 years of focus on the DOM with the progression of jQuery to Backbone to Vue, and many other libraries. At least what I've heard of the Figma approach almost sounds like the Adobe Flash/Flex runtime.
That might only make sense for applications with high levels of information density and snappy reactivity requirements like Figma or Google Docs or a web map -- not for content-focused websites. Still, it's interesting to wonder if our arguments would be more interesting these days if we were discussing those types of approaches, rather than just fighting about React vs. Svelte, or this JavaScript module loader vs. that one...
1. Wasm based UI libraries exist already, checkout makepad [0] for example.
2. Web app standards are way higher than when flash was around. I highly doubt there would be any serious discussion that would also involve vue/react as alternatives. Almost as ridiculous as asking about using unity ui vs react.
3. Flash was world class media design tool for animation and production design. People were using it to create tv shows. Figma is nowhere close in terms of alternatives like illustrator, adobe ae, etc. So it's value as a runtime for 'cool' effects would be very limited. Which is the only reason to not use html/js because otherwise there's all kinds of small usability issues (like things not scrolling right, or not working with mobile). Figma's biggest value is it's collaborative features which is really important for ui design. I think devs (and other non-designers) overvalue Figma's importance because of lack of famliarity. It's kind of like if people thought you could use google docs or notion as an ide. After all they're both text editors right, even if jetbrains or vscode is better, it's maybe 10-20% better? But of course devs know that they're world's apart, like comparing apples to oranges.
[0] https://makepad.nl/makepad/examples/ironfish/src/index.html