Earlier quoted context omitted.
What's wrong with GMail? Google stopped mining it last year: https://blog.google/products/gmail/g-suite-gains-traction-in...
All the other reasons you shouldn’t use a “free” email service. You can be locked out of your account for arbitrary-reason-of-the-day and good luck finding a contact number or email address for support. If email is important to you, you probably shouldn’t be using a “free” service.
This, in turn, means that even when considering non-free email service providers, you need some reassurance that provider will be around and will not institute crazy policies in the future. Which favors large providers like Google over smaller ones, and reinforces the lock-in.