At which point they will be so complicated, and each browser will be subtly different, that we will invent yet another abstraction layer. Devs will stubbornly only target it, and we will add more and more features until....

ChromeOS would seem to be the answer to this question, no?

I'm shocked (and a bit upset) we aren't taking the mid-way step and treating browser tabs as comparable to apps in DE taskbars.