Do they need to MITM? If they have a copy of the private key, can't they just use it to decrypt the data .. even old data for which they've only just acquired the key?
Having the root CA's private key doesn't give them access to the end entity's private keys. When you ask a CA for a cert, you only provide them with your public key (in the form of a CSR) for them to sign. The CSR does not contain the private key.
True, but they would have to do this for every single web server they would want to collect information from. Not impossible, but it'd be a lot of work.
