Most application UIs are not complicated. Most applications are just interactive documents. There is nothing special about 90% of the apps I use on my native computer that means they couldn't be rendered as HTML. The document model is fine for applications. Preferable, even.
Heck, a reasonable portion of the native applications on my computer have optional pure-terminal interfaces. If your application can work in a terminal, it can work with HTML -- and embracing a document model for your application UI would be a better user experience than whatever the heck pixel-perfect framework that everyone is chasing nowadays. That's true on the web, and it's true on native.
The problem with the web is not that HTML is incapable of being used as an application platform; it's not that we're trying to fit a square peg into a round hole. It's that both web and native developers overestimate the interface requirements of their applications, and bring in unnecessary cruft to achieve pixel-perfect layouts that are worse for end-users.
We have a square peg, and a square hole, but we like to go to conferences and pretend that our peg is actually some convoluted, recursive star shape.