I think the argument would go that if people are clicking through certificate errors and you're in a position to MITM their traffic, you can just serve them a different certificate and they'll click through the error without noticing or understanding the specifics.

IMHO host mismatch is more serious than expired cert and browsers should treat it as such

That could happen either way regardless of expiry. The only reason for an expiration date is to force site owners to cycle their certs at regular intervals to defeat the long time it takes to brute force a successful forgery.

Fair point, but I think the situation is a bit more complicated when a user "needs the site for work", or something urgent. You might have smart cautious users that feel like they have no choice but to proceed and click through the warnings since the site is most likely still legitimate