Quite arrogant to assume that an app not supporting /your/ specific note-taking workflow means it's "doomed to obscurity". It's a popular app with plenty of users, a healthy plugin community, and a team of six building it. I think that pretty obviously proves you wrong.

If you were actually interested in trying it out, you could've simply run a quick find command.

find . -name "*.txt" -exec rename -v 's/\.txt$/\.md/i' {} \;