Steve
Mostly Harmless
I get it, and this is one of those things that's as much cultural (as in national culture and also culture of the organization). I'm really just sharing my personal view. Speaking for myself, I am uncomfortable conflating a business relationship and a personal relationship, and this sensei/senpai relationship can very easily straddle the two.I see your point and agree with your last sentence that taking over "work" duties without compensation can be questionable, depending on how extreme it is. There is a difference between "assisting with lower belts" (which IMO all belts should want to help out their juniors) and "taking over work duties."
But of all the duties of the senpai I listed, teaching is the least important, IMO.
Want to say that when I say "duties" they are not really a formal job description as "senpai" isn't awarded but is rather a position voluntarily assumed by the senior student out of custom, self-imposed obligation and loyalty and recognized by others in the dojo as such.
It starts simply. Student pays money for lessons, and the instructor teaches those lessons. It starts to get muddles when, over time, the instructor imparts his ideas about loyalty, duty, and obligation (self-imposed or otherwise) such that senior students routinely donate their time and energy to the business in addition to paying for the privilege of the instruction.
The easy answer is to fairly compensate students who are taking on responsibilities beyond simply learning the art. I mean, just to be clear, it's not the increase in responsibility that seems off to me. It's expecting this (implicitly or explicitly) without any kind of reciprocity. The easy solution is to offer some kind of compensation.
Now, to be clear, if the school is more like a club... that's a very different arrangement. A lot of judo clubs around here are run as non-profits. Or dirty dog, I think doesn't charge for his lessons in the first place... runs more like a club. I have in mind commercial schools like your typical karate, BJJ, or TKD school.