How does your model fit...

My model is purely only targeting the food services, using it to target other professions is not its intention and won't work as a result. Just as you would expect different levels of attention and service from different professions (nurses would pay more attention to the "servings" given, regularly check up on your status, etc.), you'd set aside different ways to approach tipping to the respective professions as well.

I do agree with you that the "rules" are only guiding principles given the current nature of how people collectively decide it, but I don't think it's purely arbitrary like that either. There's certain expectations to tipping or giving back a certain reward (compliment, thanks) that doesn't change even with the cultural expectations changing over time.