Which is provably false. (1) AppleID email address doesn’t have to be iCloud mail; they’ll be forwarding to my mail server in the first place. (2) iCloud account is free, whether or not you decide to stop using Apple devices doesn’t affect that. (3) They said Sign in with Apple will be available on non-Apple platforms. (4) I doubt third parties can’t offer an option to decouple Apple entirely if you so choose.

You can also provide your own email address. The choice is yours. Which threat do you think is greater?

I think Apple shutting down your access is a legitimate concern. Yet Apple is the only one who has taken any steps to protect the customer here. The reality is we are unlikely to see other major competitors to Apple find a better alternative because their entire business is built on exploiting user data.

And 3rd parties who create a similar system are unlikely to be successful because they don’t have the clout to convince websites to incorporate them.

Until they remove the forward to your inbox?

Your gmail, Hotmail, yahoo inbox ! Who can disable your account for some undisclosed slight

If you can see the forwarding address, then you could probably somehow contact the company to switch your account. Not extremely secure and would require more validation than simply knowing the random bits of the address, but doable.

You can access iCloud email through IMAP/SMTP on any operating system you want, just like you can access the calendar through CalDAV and the address book through CardDAV. Apple isn’t Microsoft.