We’ll see how it works out, but my guess is the end result is going to be the same as with every other platform flutter has integrated: it needs to perfectly replicate the behavior of every native UI component on every mobile os / browser otherwise it’ll feel something’s wrong. For example scrolling, right clicking, resizing, zooming, button click, link colors, css animations, are all behaviors that could have various subtle behavior variations depending on the browser and its content (not to mention extensions such as ad blockers).
Also, integrating with other js components will likely be impossible as well ( try to integrate the google maps js component into your layout... good luck)
And browsers update way way more often than mobile os.
The UX you describe seems to fall in the 'naughty websites' category in my book, indeed. I think there's a "uncanny valley" of atrocious UX where it's "almost like the real thing but not quite" and it feels very disorienting. I'm pretty sure that wouldn't fly with most projects.
This and what you mention about js components imply that, what, we should expect each and every useful project out there to increase manhours to make some interface for flutter? I mean, short of a fits-all transpiling-middleware that takes in Js to output whatever Flutter needs to work with it, it's just not happening.
I'm not sure Flutter is as general a framework as we make it out to be; that it targets many platforms doesn't mean it targets many use-cases, let alone all of them. Afaik, it's a heavily 2D-graphics oriented solution, great for design-rich projects, and I had failed to see the restrictions (apps being their own little worlds) until you outlined what it means when translated for the web.
For the web I'd bet on some combination of wasm over this any day, tbh, combined with progressive web apps it seems a stronger, more general/opened proposition.
Also, integrating with other js components will likely be impossible as well ( try to integrate the google maps js component into your layout... good luck)
And browsers update way way more often than mobile os.