> Emails already support updating by including external image references inside the email.

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.