Hello, and welcome.
If you try to send 1000 emails with SMTP, you will automatically figure out why SMTP is not a good idea for bulk emails. It will be so slow that you will need a background process to send them. It can take from a few minutes up to 10 minutes to send 1.000 newsletter emails with SMTP. This generally takes under a fews seconds with Mailgun HTTP API. Also, sending bulk emails with SMTP is blocked on most of the hosting providers.
Regarding to giving that option anyway with warnings etc.: As far as I understand the general principles of the development of Ghost, Ghost team doesn’t want to spend their limited effort for the features that they don’t use or find correct. Even with warnings, confirmation dialogs or so, people will come with the issues of SMTP newsletter sendings here and there. At the end it will not provide a smooth experience.
The proper solution to this is having an adapter for newsletter emails in Ghost. Ghost team is open for PRs (There is an ongoing effort for Postmark Integration). But it needs to be aligned with the general experience of Ghost.
Another possible workaround is using a “custom Mailgun API proxy”. By mocking Mailgun API endpoints (Ghost doesn’t use many endpoints of it), and setting mailgun api address as your custom domain, you can do whatever you want when Ghost tries to send a newsletter and get the state of it. I actually use that approach, to count sent emails in Synaps Media, not for replacing Mailgun. But I can see, it can be easily used to send emails with another provider.