I think the other answers are misinterpreting your question. A user is held "hostage" because unlike fastmail where the format is word.salad@yourdomain; if you ever wanted to ditch iCloud completely, you'd have to go through every single account you used it for and update the email. As someone who has shed his gmail account I can tell you it's not easy to update email address on every site you've ever used it on. The issue people are raising is that apple doesn't let you use your own domain, and generate random emails at that domain. Not my complaint, but I can see the perspective.

Marketing turds can't just block registrations from all of @icloud.com.