You can usually automate the renewal or buy for a long period.
But mistakes can never be completely avoided.
What if you accidentally transfer this domain to someone instead of another domain you sold them? What if you accidentally lose access to the registrar's website? What if.....
I actually had this happen to me. My credit card was compromised, the domain (which was registered 10 years ago) was tied to an old e-mail I never use, so I never got payment failure alerts, and didn't notice it was expired until a few days after I stopped receiving e-mails and the domain had been bought by someone else.
It's a huge pain to deal with, since it's 100% unrecoverable since there's no one to really appeal to and you're just forced to update your e-mail everywhere.
Many (though not all) TLDs employ a "grace period", where the domain stops working but you have some time (usually 30 days) to renew it before it's really de-registered. This is to prevent exactly these types of scenarios.
I had a similar issue where I didn't get the renewal failure emails. Fortunately no one else grabbed the domain, but there was a window of a week where someone could have! I still use my own domain for my email, but I can't help but feel like maybe it's not the best idea.
What if I want to change the domain? My org changed its domains twice in five years I've been there, and porting e.g. imports in all of the internal Golang porjects was not a funny experience.
>What if you passed away and someone wants to access your old data by getting your old email domain Oo
What exactly do you think happens to anything of yours after you die if you don't prep for it? With many organizations or property your heirs can use a death certificate or court order to get stuff, in other cases they may just be out of luck. You may even wish it to be so, do you actually want them to have access to your old data? All of it or only some? If you actually care then like anything you need to prep for that while you're still alive in any one of the numerous ways available. If you don't then tough shit, that's why there is constant reminders about setting up wills/trusts and keeping relatives/friends/colleagues in the loop as needed and so on and so forth.
