Who owns the domain here? I use a custom domain with Gmail Workspaces, but I own the domain, and therefore control the MX records. The only thing I'd lose in a switch is stored email messages, but there are ways to download those.
It will be a huge pain in the ass trying to do it for my grandparents though.
And at this point, the pattern is clear. Google Business products are free until they are not. Building them into your infrastructure is taking on technical debt that you will have to pay down in the future, either by actually buying the product when they decide it is no longer free or by switching to an alternative.
I still run into websites with a big API disabled rectangle where their Google maps integration used to be.