But you would have to invent or assume a format for that .txt, and markdown is easily rendered if needed

Invent a format? How about, "The system overview has been illustrated in images/system-overview.png"

We use markdown because it is plaintext and accessible. Oh, but you have to convert it yourself. How? Well every project is different but I'm sure you can find a Makefile somewhere in the project. Good luck!

Again, the point again is that markdown is not readable alone.

If it is, then a .txt is strictly superior. If it is not, then never, ever, present it as the final product. Always ship the generated artifacts. The only point you want to actually see the .md file is if you want to make changes to it. Is making changes the primary usecase for your markdown files? I'm going to assume, no.