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