Everyone I know who isn't tech-literate uses Gmail because (a) Gmail is/was better than their ISP's email in many ways, including UI and spam filtering, and (b)

Everyone I know who isn't tech-literate uses Gmail because (a) Gmail is/was better than their ISP's email in many ways, including UI and spam filtering, and (b) it's hard to switch off without dedicating a few hours to changing your email on a bunch of services (and there are a lot of services that don't offer email changes at all) - also, if someone hasn't use a password manager religiously, chances are they don't even have a convenient list of all the services they're signed up for.