Email is expensive because the tacit permission of the receiver is not handled end to end.

Yes, there is actually a way to solve the end to end tacit permission problem, which involves cryptography (obviously). And yes, there is a way to make it ergonomic in a way that users could actually use and understand it well enough that the below-average user would do fine with it.

With this problem solved, all of the anti-spam stuff and all the trouble that comes with it becomes obsolete. Until this has happened, email is going to continue suck a lot.