> That said, Comcast is big enough that it might be in cahoots with at least one less-than-scrupulous CA (or might even be a CA; I don't follow these sorts of things closely enough).
The moment that was discovered, the CA would stop being a trusted CA.
There's a lot more appetite these days for enforcing requirements on CAs, ever since CT started going down the road towards mandatory, and ever since misbehaving CAs started getting forced to implement it immediately. Intentionally MITMing TLS on the broader Internet the way this thread is talking about would be a fairly quick death sentence.
Also, given the existence of Let's Encrypt, there's much less reason to be using a paid CA, and planned migration to a new CA provider isn't too much to ask. There'd likely be some work within browsers to provide clear error messages, and sites would need some amount of time to migrate, but I think we're talking days-to-weeks before there's a warning banner on the sites and months at most before the CA is dead.
The moment that was discovered, the CA would stop being a trusted CA.