It's not like Markdown is some rare, unheard of format. I think it's the most used file format for technical docs / content / notes these days, and it's here to stay (because it's simple, it's easy and it's portable). Besides, most well-known note-taking services and apps are based on Markdown, or they export losslessly to Markdown.

There are probably 1000 markdown editors out there ranging from a simple textarea that only lets you write text, to Emacs, and something like Msword on steroids. The nice thing about an open market is that when you no longer like a product you can just switch to another one.

I however understand your issue with privacy as any app can just steal your content without you knowing. The solution for privacy is to sandbox/jail the app into an environment where it can just access a limited set of resources, for example you can use an apparmor profile where the app can just access a special folder, and disable all networking. That special folder could be your google drive mount.