Yes, but if one goes out of business you can buy your domain from someone else! Or even transfer it, if you are warned a nonzero amount of time in advance.
Being able to keep your address across service providers being the important part here, that way your online life doesn't get in a huge jumble when your provider goes away.
I've hosted email for myself and some other users for decades now. We all own our domains and aren't getting any younger. I'm a little terrified of the responsibility I've taken on and have no idea what I'll do when we start dropping like flies. Renewals will lapse, allowing squatters to snatch up our domains, read new incoming email and impersonate us. I'm not thrilled with depending on third parties, either, but now I wish it was someone else's problem who might outlive us and exercise a small degree of ethical oversight. Maybe there's a business idea here...
That's fair enough, as long as you would trust that business to not misuse your address space for other things. Same level of trust one would put in a regular email provider, I guess.
Being able to keep your address across service providers being the important part here, that way your online life doesn't get in a huge jumble when your provider goes away.