I get that it would be (mostly) great from a business perspective but much less so from a users standpoint.

Email is a sacred medium. It is an archive of conversation and if messages were able to be modified that essential behavior would go away. Imagine if all of your emails could be potentially altered at any time. There would no longer be trust in the medium.

Email is (relatively) safe. That would go out the window if a remote user had control of the content of messages. First, what guranteed would there be that a nefarious person doesn't change the target of a form or link? What happens when the content of a brands message is changed without their knowing? How long are messages modifiable? Can JavaScript be injected? We all know how far downhill thigs could go in these cases.

Overall the benefits don't outweigh the costs.