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.
Yes you can do word.salad@yourdomain with fastmail but that ius no different to other mail providers.
What the above comments re fastmail is about their masked email service this gives addresses like <random>@fastmail.com so this is the same lockin as Apple with the same benefits of noone is going to block that domain.
I mean, that is no different than any other provider.
If you want to degoogle, you still have to go to each site and change your staticuser@gmail.com individually.
Most users already do not use custom domains if that's the catch.