Write down all your accounts and hope they'll send you a warning in advance, once they decide to go out of business. Then you're going to have to change all the accounts to a new email.
I'm pretty sure the big paid providers aren't going to fail you withoit a warning. And gmail etc are too big to fail. That's going to wreak havoc with a lot of other users... Though: If they decide to ban you or delete your account... You're going to be in big trouble. That regularly happens to people.
Only alternative I can imagine is to run your own email service. If you own the domain and server, it's your call. But you have to pay attention to maintain it and not get hacked etc. That would be another way to lose email accounts. (Running a mailserver is more complicated than hosting a website.)
I've had that discussion before, here on Lemmy. From my experience, like >90% of people will tell you not to do mail yourself. And there is a reason to it.
I mean don't do it if you don't know about DNS, all the added antispam like SPF, IP blocklists and how the big players handle that. And don't make any configuration mistakes and become an open relay for spam. It's certainly doable, though. (With the proper Linux admin skills.)