Serious question: Why would it be preferable to have something like this in the browser? The thought of having to keep track of a browser tab that you might accidentally close and lose all your work doesn't sound great.
I would love to see a DAW where randos upload just a track—maybe a drum track. Other's can "create a branch" where they jam along: add a bass track. And let the jamming, branching, songwriting commence.
Doing this in a browser is ridiculously inefficient.
But if you just want a limited-track scratchpad for virtual jamming, it's the most accessible option.
It's not a useless thing to be able to do, but there are few people who seem to want it at the center of their music-making activity. At least that is the way it seems to me. It initially appeared to be an immensely cool new capability, and in many ways it really is. It just isn't a thing that many people want to do.
The sort of live sound mixers you're describing just send control signals between the pad and the mixer, no audio flows to or from the pad.
... and it is still different from virtual jamming, which more or less definitionally requires a WAN. If you're all within wifi range of the mixer, you're actually jamming :)
The allure is that the web is the most open, most stable and the most cross-device platform we have. Almost anything that was made for web still works today, with Flash and Java applets being the two big exceptions. Following the Lindy effect the self-contained web apps of today will still be operational far into the future.
Contrast this with Android's pathetic record of constantly breaking backward compatibility and restricting what software the users can even run on their devices.
Compare that to a win32 desktop app that will almost certainly keep working indefinitely without any changes. Plus you can use proper files for storage.
You are right that by the nature of web the long term stability is not guaranteed. The ecosystem could do more to encourage decoupling and graceful degradation. About 95% of modern web is competing in sharing your data to even more "parties with legitimate interest".