My use case is also to receive e-mails from non gmail/outlook users. In fact if I check my logs 1.5% of messages came from google's smtp service and 1 from outlook, out of last 15000 emails.
So if big provider drops 95% of incomming emails from non-big providers into black void, their service is less than useless to me, and their quality would be at par with small providers that can deliver 95% emails successfully.
It's symmetric in the sense that if Alice can't communicate with Bob, then Bob can't communicate with Alice. But network effects break the symmetry. If you have a big provider you can reliably communicate with 99.99% of people. If you have a small provider you can reliably communicate with 95% of people.
*Numbers made up to illustrate the point.