Going from shared memory to protected memory was a big step for multitasking on the desktop, and since web applications are more and more complex, the same move needs to be made with browser design
This is a VM/Renderer implementation detail, so that a bug in the browser itself only impacts one tab, but it doesn't do anything to actually improve the current programming model. If you were confident enough in your browser to securely and reliably handle all input, then there is no advantage to using multiple host OS processes.
C'est magnifique, mais ce n'est pas l'Informatique. -- Bosquet [on seeing the IBM 4341]