This is incredible!
> enough maturity to implement essentially an entire operating system / virtual machine AS the browser
Yes, an operating system-- but one without user-facing persistence! Only per-application, networked, silo'd persistence, where the user sees a rendered form of data but not the data itself.
IMHO these two facts, that
a) Web browsers are BETTER than Linux, Windows, etc as an OS in a very important way, package management
and
b) They're missing one of the primary features of an OS, persistence
overshadow any other facts about them. What a strange tool they are.