I honestly think that most people don't know why it matters. 20 years ago I was happy (and proud) to have gmail, and didn't see a point in having my domain. Until I chose to move away from gmail, and then it all made sense.

I have an example: many (most?) universities give you an alumni email when you graduate. Some offer actual hosting, but others just ask you to redirect. It is free, and for many universities, graduates would trust their university more than gmail, right? And it wouldn't lock them into gmail. Still, none of my friends from university use it. Most use gmail instead.

Now tell them: "Look, if you use gmail, then Google can read all your emails. You didn't give a shit 4 months ago ("I have nothing to hide"), but now you've heard of random Canadians and Europeans getting deported or declined entry in the US for random reasons, including one who wrote stuff against Trump. How do you feel about the US reading you emails and deciding whether they should deport you or not based on that? If you controlled a domain, you could move away from gmail. Now you can't. Also know that if they can read your emails, it means that they know the flights/hotels/trips you booked, whatever you bought online, and they can just access all your accounts everywhere."

What will they think? "If I was to do it again, I would still use gmail" or "Would actually be nicer if I had been using my alumnus email all along"?