Additionally, you can't send email. That's also a pretty important part of the whole "email" thing. Any ongoing discussions will just be cut off.
Seems pretty walled garden-y to me, considering the entire framing of this is that nothing bad was done with said email. It was because Apple has a unilateral switch that turns everything off if anything bad happens to an associated credit card. That is to say, if your credit card number is stolen, completely outside of your control, this could happen to your email. I'm not aware of any other email service where that is the case.
Federated email just generally sucks, insofar as email addresses are expected to stick around indefinitely, yet are almost-always associated with some service provider you might eventually no longer be a customer of (for whatever reason) and with "owning your own email providing" being nigh-impossible for deliverability reasons.
Also, people's emails to you wouldn't go into a black hole; they'd start bouncing. Which would mean that those people would realize that they should be reaching out to you some other way.
(In fact, properly-coded 2FA systems should recognize an SMTP bounce and use it as a signal to temporarily reduce the level of authentication required to reset 2FA. Things like this are why it's so hard to get 2FA right.)
Yes, if my non-Apple email service "locks me out", then the same would happen as here. The point however is that:
1. Usually this would only happen if something weird specifically happens on that account. For example, if I am paying for an email provider and my card is declined. Notice that with the Apple case, a problem in something completely unrelated to email can cause this (including apparently the card being flagged for a fraudulent charge on some unrelated service).
2. Even then, usually the next step is I am told that that happened. Notice that in the Apple case they won't even tell you what the situation is to give you a chance to remedy it.
3. Finally, you then almost always have a way to remedy the problem. For example, by giving them a new credit card. Notice in the Apple case they insist that the account is disabled forever.
Do you see the difference? It's not in the "what happens if your email stops working" part, it's in the "what leads to your email not working anymore" and "how do you fix your email" parts.
Do you have any evidence Apple has done that here?
Uhm, conjecture? The Reddit poster complained about app/media store lock, if they lost something more important like iCloud with photos and email that'd be a totally different story instead of "I lost some movies".
Second, they only ever said they asked the rep about their purchase content. Nowhere in the thread do they imply they lost anything else.
Second reason I use Thunderbird (or whoever wants whatever else) is that I have 10 or 20 mail boxes like separate for shopping separate for more important stuff separate for stupid stuff.
You don't have to have a single mailbox with a single provider.
...but you can use iCloud Mail through regular POP3/IMAP and thus have a local copy of all your mail. (And, as far as I know, this is what Mail.app itself does, save for some push-notification optimization; am I wrong here?) There's no walled garden here. It's email.