How is that different from - I saw a price for a flight on the website, but now its gone. If it was going to change, why did they show it to me?
AMP for email makes json requests proxied by the email client. Your client can decide to only make these requests once and then keep them stable or to show you a version history of the state of your email upon every time you opened it.
I think this is not comparable. If an email links to an external image, the external image is not the email, it's a link and behaves as everyone expects a link to behave. The email itself is immutable.
This may seem to you like hair-splitting, but I don't think it is, since blocking external images is a common thing for people to do anyway. Doing so doesn't make email useless.