I'd hardly say this is a feature. It's an artifact of the fact that once upon a time computers were expensive, so many people had to share a server with multiple email accounts, and a netop had to run this server. The fact that Alice is beholden to example.com helps nobody but example.com. In an ideal world, Alice would not have to couple her infrastructure with a blocklist and Alice would have the choice of blocklist herself.
Spam is inevitable. Opining about ideal worlds is fun, but it's also irrelevant because that world is gone and it will never come back because it can't.
Alice is not beholden to example.com, she can use any hosting provider she wants. Moving providers isn't even that difficult. It's also totally irrelevant, because again spam is inevitable.
