And one day you've had it with Apple's latest user-hostile shenanigans and switch to Linux. What now? Do you just keep paying for iCloud+ forever?

In my experience the overwhelming majority of services permit me to change my email address.

Of course. But I have hundreds of user accounts, as probably many people do. I would not enjoy changing all those email addresses.

wouldnt this be the case for any vendor you choose?

Indeed. But some are easier to change than others. I switched my e-mail provider, and it took all of five minutes to launch the copy of my data. Since I kept the same domain, everyone sending me e-mails didn't notice anything.

With Apple's approach, I'd have to go through each account and move it from something@icloud to something@new-domain.

However, for people who don't want to mess around with custom domain names and e-mail providers, apple's approach is very practical. You just need to tell it to "hide your email" when you register somewhere and you're good to go.

yes

As someone who uses both, I much rather prefer aliases to hide-my-email for the more important stuff. For one, I can choose the email address "username", which I cannot with Apple's solution. Plus, what happens when I move on from Apple to something else?

But aliases can be easily mapped back to your normal email address, unlike Apple's which are opaque. I, too, am afraid of vendor lock-in though. Sadly, couldn't find a good alternative yet

There's no solution to lock-in because there must be some massively shared domain that the email address exists on for the anonymity of the service to properly work. However if you are simply looking for an alternative to Apple, Fastmail offers a masked email service too.

Not sure where you're coming from - my original email address is not being shown in headers, so those seem fairly opaque. Probably depends on your email provider?