If that were the case, obviously it wouldn't work without some infrastructure changes.

But imagine an app where each function (like, say, the levels filter) was downloaded as needed and cached. That might be possible sooner rather than later.

I'm sure there are plenty of reasons why that's a non-starter. But I don't think that it's impossible to have an application of whatever complexity running in a browser... I mean, I remember when doom came out and I had a hard time running it on my machine without tweaking the configuration. And now...