However useful, I have a feeling that is more likely to be the exception rather than the rule and should therefore be prioritized accordingly

I don't see how Apple could've made it any easier.

1) Select any file.

2) Change the app in the "Open with" dropdown.

3) If you want it to apply to all files click the "Change all" button.

The fact that the upstream HN comment has to explain two identical-sounding options that do different things should help you see what's wrong with the current impl. Your steps don't work with the "Open With -> Options" context menu like they point out, but rather only with the "Get Info -> Open With" options. Not the best UX.

For example, I can imagine if there were instead two radio buttons in "Open With": "Always for this file", "Always for this extension". All in one place.

Doesn't work when Xcode is the default app. I use macdown for mathjax files for math.stackexchange.com, and I have to use "open with" to choose macdown every time.

In old days Mac OS files had two properties: file type and creator type. File type could be like "TEXT" and creator type was a unique code for the application that created it (they were registered at Apple, I believe). All this was purely internal, users never saw such things: Mac file names normally didn't have an "extension", it was just "1995 Report" or something.

As a result it behaved like that:

- When you opened a file, it opened in the program that created it.

- When there were no such program, you could still open it in another program that claimed to understand these files. I don't remember how it was implemented, I think there was a dialog asking if one would like to use another program with some choice (probably the first one that fit).

This was rather handy with formats like "Encapsulated Postscript" in desktop publishing: this format supported both vector and raster images, but one normally used different applications to manipulate them.

This stayed for some time after the move to Mac OS X, but now I believe these things are gone or not used at all.