how is the user being held hostage? you can redirect hide my email addresses to any domain
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.
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.