I used to use Ghost, so don't discount me as some "HN commenter." I switched to Markdown not for the cost but for the portability. It simply seemed easier to edit Markdown than to go into Ghost's editor for basically the same thing.
Ultimately your not wrong, but I think Ghost is a great platform.
If anything I like creating tools that I can share with others. If I want to invite friends to post on my blogs it'll take 30 seconds to provide a login and creating posts is very easy with Ghost.
Getting non technical people to understand Git is a massive challenge I don't want to try again.
I see. Personally I don't let others write on my blog so I don't have the same use cases as you. If I had to, I'd ask them to just send me the Markdown file or text itself and I'd submit it via git myself.