Ghost Pro (which is managed hosting service maintained by core Ghost team) is happy enough with Mailgun and publishers using this (and other) managed services don’t have any issues with Mailgun. I understand the frustration that you have as a self-hoster. But creating an adapter to allow other type of bulk mail services need a big amount of effort. Ghost team said they are open for community PR for a proper newsletter adapter mechanism but no-one made this yet. Working on a feature that you never use (I mean core Ghost team here), is specially difficult. So I understand the position Ghost team takes here.
Besides that, there are some other workarounds to use other bulk mail providers with current Ghost. You may want to check GitHub - tilak999/mailgun-ses-proxy: Proxy server to use AWS SES instead of mailgun in ghost CMS · GitHub project and @jannis mentioned another PoC he is working on on his blog. If you don’t care email deliverability stats in Ghost admin, actually this proxy method is considerably easy and safe solution.
I hope this helps.