What if your domain registration lapses and someone else grabs it?

Registrars do warn quite a bit but indeed you can add a yearly notification 1 week ahead in your calendar.

What if you can’t afford the cost five years from now?

You are in quite deep trouble then because the registrar itself is relative cheap, e.g $10/year. It also does not seem to increase significantly. If you can't afford that you probably should focus on basic necessities first. If you are serious about it though, just like with the yearly notification, set $1/month just for this.

What if you just don’t like the domain name someday?

I mean... you change it? Just like when you went from person@gmail.com to person@mydomain.tld . That process is a bit annoying but as you've done it once, it will be easier.

All of these reasons will be problematic and some can result in identity theft and significant fraud. It’s definitely not a decision to be taken lightly, particularly if you have a lot of online accounts.

It's not a light decision BUT it's also not such a big deal. If I want to go back to person@gmail.com I can just do so any moment I want (well person-something@gmail.com to be precise). I will keep a 1 year grace period for the transition, start with the most critical accounts first, e.g government and banking then social media, then random accounts based on my history. It's annoying but it's a matter of hours over few weeks at most.

The only challenge is to be methodical and giving up on the idea that you'll update 100% of the account. Getting 99% of the account that truly matter is enough IMHO.

PS: for actually sensitive data, and assuming you somehow didn't manage to get the grace period YET still are smart enough to think ahead, multi-factor authentication will keep your accounts safe. Honestly I don't think the overlap though between somebody who cares enough about that AND let's domain expire is very big though.