It ossifies every layer of the stack below it. Once you have a practical monoculture, user demand effectively cements the entire underlying operating system design to cater to the browser implementation. Fine if you're selling Chromebooks, but fatal for innovation.

Once the operating system is encased in amber, only a massive budget can break new ground -- this is why things like Plan 9 from Bell Labs will never take off. It's why Haiku is importing more and more BSD code instead of pursuing their own paradigms. It's why the only realistic chance for anything new is Google's Fuchsia; they're one of a very few organizations who can afford to innovate in this environment.