The attack surface of the kernel is much larger than the attack surface of a browser. Not saying this is a bad idea, but it definitely opens up much more room for much more serious attacks.

The browser already mitigates this via the normal system call sandboxing on the various OSes.

And something like this already existed, 32bit chrome would play games with the LDT on OSes that allowed that to build a better sandbox. It's just that the LDT more or less disappeared on x86_64.