Damn I hope I never have to leave gmail and bother with this self setup crap. Would you self hosting guys feel silly if gmail existed in 30 years and you wasted all this time doomsday prepping for nothing?
You could have best of both worlds: Hook up a private domain to GSuite. If Gmail starts sucking the next 30 years you can move it to the next best thing :)
GSuite isn't free anymore for most though (only for grandfathered in domains). For some the cost is negligible, but it depends.
But ya for me the piece of mind is worth it. The serious risk, as you allude, is probably not gmail disappearing, it's gmail getting worse in any number of ways.
I stopped using Gmail because I decided email was too mission critical for me to trust Google with. I'm now with fastmail and appreciate the fact I can port my email elsewhere easily should I choose to do so. No regrets!
But I'd be pretty fucked if google decided to ban me back when my gmail address was the email address I used for everything.
As for self host, you know that person said they pay for email right? We log in to fastmail the same way you log in to gmail. It's no hassle just a low cost instead of free. Iirc getting it to use my personal domain was a copy and paste job that took like 15 seconds.