The benefit you give (be able to "cut" a kana in the middle) is really weak, I've never seen anyone being confused by that when learning in kana.

This is completely nullified by all the drawbacks of using romaji while learning and they're well known already.

The only reason to use romaji for Japanese grammar is to explain the concepts to someone who has no interest in learning the language, just for their general knowledge.

You glossed over my point. I’m not using it as a “crutch” for reading. I’m using it to have notation for the stem — the thing before -u. I could choose alternative notation with kana (e.g. just always using the -u ending, or the idea of variable stems like i-stem and a-stem) but then the visual “gluing” wouldn’t work. Which is the whole point of mental model I’m communicating. It’s fine if you don’t find this mental model helpful but it’s the point of the article.

I’ll be honest that I also wanted (as a challenge) to write this article so that a person with zero Japanese knowledge would be able to correctly conjugate almost every word to every ending. This is more of a teaching drill for myself though but it’s another reason for the romaji choice.