Your response seems to imply those people are irrational, but it's really just different priorities. Yeah, you need to make sure you don't lose the domain. With Google/etc, you need to make sure you don't break any of their usage policies across their suite of apps, etc. Neither are super likely to happen. Neither are impossible, either.

(Update: as it happens… https://news.ycombinator.com/item?id=49548452)

Personally I like having a custom domain as I like the idea of being able to move between providers. So far, over the past decade, I've hosted my email with Google, Fastmail, Hey.com and then Fastmail again. I like being able to move it around if I find one provider better than another. Others won't care, that's fine too.