I agree the Church of Plain Text is indeed strong, but to the point of proprietary formats, there are happy mediums.

Take Ulysses for example. It blends plain text Markdown with excellent in-app features to give you a much more rich experience than a run-of-the-mill text editor.

Ulysses does it’s job exceedingly well at containing your text while also adding methods for organisation, formatting, themes, tools, and metrics to feel proprietary but remain closer to simple plain text.