Earlier quoted context omitted.
As someone who is fluent in Japanese, the thing is that this is not an example of a method that works well, it’s an example of a beginner coming up with an overwrought headcanon (while calling it “simple”, nonetheless!) that ultimately will hinder their understanding, as I will detail elsewhere. If this was a similar post about programming, it would not get upvoted here because more people would recognise it for what…
How is this my headcanon when this is literally how it works linguistically? Please provide a counterexample.
Personally, as a beginner/early intermediate learner, what I have found works is just to allow my brain to learn this over time by reading and understanding more an more sentences. I used Genki and am aware that it thought this badly. But it was in the 3rd chapter and by the time I reached the 6th chapter I had gotten an intuitive idea anyway (とている just sounded wrong and I couldn't explain why). And I wasn't doing much output anyway so it didn't matter that I maybe confused a couple of verb endings as I recited the 2nd writing exercise og the 5th chapter of my textbook.