I think this is actually brilliant, but not because of the business proposition (percentages, etc.).
I believe "stem" has a double meaning. Sure that's what they are calling the four touch sensitive lines, but it also has an educational definition (science, technology, engineering, math). Check out the company making this (Kano)... It's nothing like one of the big tech/audio firms. Look at the device itself and how simple and toy-like it is. Listen to Kanye's vision of future society/utopia. This product clearly has a strong educational vision. It's about making audio tools more available and accessible outside of a full blown computer/mixer/program. It's about fostering creativity and curiosity in younger people who all the more these days are just trained to be mindless consumers rather than creative producers.
Admittedly Kanye is still crazy, don't hear me wrong. I don't think this is going to be a big money maker for him. However, I find the educational aspect of it to be absolutely fantastic, and I hope it's just v1.0 in a long line of successors.