That doesn't protect any active sessions; if you assume the attacker is capable of exploiting the browser, then they can just exfiltrate an active session for the user for any domain and bypass the login mechanism entirely. Done. This "reset state" approach can only protect against that if you completely wipe after every sensitive interaction on every unique same-origin policy, but at that point you're just doing all the hard work yourself.
Obviously I'm not saying 2FA isn't good, and doesn't mitigate some clearly related attacks like raw credential theft (whether or not the browser is exploited, obviously.) My position is just that browsers-in-VMs is a mostly roundabout threat model whose actual benefits (such as some semblance of filesystem isolation) can be achieved other ways. The things these approaches can not fix are otherwise systematic issues that require major redesigns to achieve.
Obviously I'm not saying 2FA isn't good, and doesn't mitigate some clearly related attacks like raw credential theft (whether or not the browser is exploited, obviously.) My position is just that browsers-in-VMs is a mostly roundabout threat model whose actual benefits (such as some semblance of filesystem isolation) can be achieved other ways. The things these approaches can not fix are otherwise systematic issues that require major redesigns to achieve.