> It's even easier to do, because you don't have to trick a CA in creating a duplicate key.
In some ways it's easier with PGP, yes.
But in some ways relying on a possibly-hostile CA is worse: if the software doesn't really give the user any visibility of key changes, then the impersonator won't even need to social-engineer the recipient with "whoops I lost my key". Instead, the duped recipient will just see "Signed by Big Trusted CA" with a shiny green padlock, and will think everything is fine, even though the key under the hood has changed.
It's even easier to do, because you don't have to trick a CA in creating a duplicate key.