It wouldn't just be the browsers removing the CA. There would be a strong incentive for websites to switch as well, particularly foreign ones, so you'd find a mass exodus anyway, even without browser support.
Browsers don't have to turn a root CA off all at once, either. They could start by turning off Extended Validation for the compromised CA, or they could release a statement saying that if they don't get guarantees this won't happen again, they'll remove the CA in a year's time. They could allow connections, but change the SSL icon to indicate the certificate has been compromised. Browsers have a lot of options to put pressure on root CAs, even without removing the cert.
Browsers don't have to turn a root CA off all at once, either. They could start by turning off Extended Validation for the compromised CA, or they could release a statement saying that if they don't get guarantees this won't happen again, they'll remove the CA in a year's time. They could allow connections, but change the SSL icon to indicate the certificate has been compromised. Browsers have a lot of options to put pressure on root CAs, even without removing the cert.