Granted I am probably clueless about the "real world" of aviation and that's why I'm here, asking questions and learning about this.
However, you seem to be too focused on what the instructor is teaching... which is obviously useless in an airline environment. My point, which you seem to still miss, is that this is about evidence of the desire to and ability to teach.... are you saying this is also useless? I say it isn't, and if so, that'll make this "real world" of aviation close to the "real world" of healthcare where effective teachers are very valuable. Granted basic PPL FI pay isn't reflective of the value they bring, do you say it is the same for TRE/TRI in airlines?
As for advanced ratings, well, I think this thread already established that flight instructing pays a pittance compared to other types of jobs so if you think I was referring to getting other ratings while relying solely on FI pay, you've missed a few posts.