You are correct that there can be uses for the .md extension. I should have said, for my use case I find it unnecessary.
I want my notes to be a set of plain text files, with the extension .txt.
I believe .txt is the best extension choice for my notes. .txt is likely to outlast Markdown. .txt better expresses what my notes are: at core, they are a collection of plain-text files.
As Markdown syntax is perfectly readable in plain text editors (this is the point of Markdown), I don’t lose anything by using Markdown syntax in my notes, and gain much. This is an excellent property of Markdown – it can be treated as plain text!
When opening my notes in editors that have extra features for Markdown, I tell those editors to assume Markdown syntax for all notes in my archive. As Markdown is backwards-compatible with plain text, this doesn’t break when some of my notes aren’t using any Markdown syntax.
I would like to be able to tell Obsidian, “treat these .txt files the same as any other Obsidian note”.
The community plugin was very interesting, thank you. But I think it doesn’t allow me to link to .txt files without the extension, and view them in the Graph view. Using this extension feels like .txt files are second-class to the .md notes.