Well, then that mail is going to get legitimately marked as spam when I didn't request it, I have no business relationship with the sender, and there's no other

Well, then that mail is going to get legitimately marked as spam when I didn't request it, I have no business relationship with the sender, and there's no other way to make it stop. And the user is still getting silently charged because it's going to my inbox, not theirs. Unsubscribe is the alternative to the spam button. If you don't provide it, you are asking to be marked as spam, no matter how important you think the email is.