> Flash apps were also "true web apps"
Not really. They were not portable. NPAPI was not a web standard in any meaningful sense (hint: the first N stood for Netscape).
> That's why there has never been a UI toolkit as successful as the web.
The web's success in terms of active users and in terms of deployed content is astounding, certainly. I think I'm relatively well placed to understand why, and I don't think it's because the web restricted developers to one scripting language or one set of controls. I think it is very much _despite_ that.
That is exactly defeating the point. Flash apps were also "true web apps" that only happen to use the "limited control set" of just <embed>.
> It's basically how every other platform works
That's why there has never been a UI toolkit as successful as the web.