No, it's not equivalent to plain text. If they had access to certificate authority keys (or the ability to compel CAs to issue certificates on demand), they would still need to pull off an active man-in-the-middle attack to be able to decrypt communications. Practically speaking, it seems like this would be harder to accomplish on an Internet scale than simply intercepting communications. So using HTTPS as much as possible probably helps, though you're correct that if you want to be completely certain your communications are secure you should not trust the certificate authorities.