If the Flutter dev experience is good enough, this will eat traditional web apps slowly. In a way, the web was designed for this sort of scenario: a delivery mechanism agnostic of the client implementation. We've gone from pushing server-generated HTML to megabytes of JS + SPAs, and now a WASM/Canvas hybrid seems inevitable.
It all comes down to how well these perform on mobile devices, what the dev experience is like, and how the end product feels. There's still a lot up in the air, but, I'm honestly impressed with the progress shown by Flutter here. There's still a host of considerations unaddressed (including accessibility) that I hope they have an approach for.