Are you implying dancers or painters do not do their own solitary practice to improve their ability for its own sake, just to be better dancers or painters because, maybe, just maybe, they sincerely enjoy dancing or painting? Are you implying that everything a dancer does or a painter does is solely with the goal of displaying it to others for their pleasure? Are you implying that martial arts does not have its own depth and an ability to bring people together or be pleasing to others? Are you implying that one can get good at aiki without highly specific tanren? It sure seems like it.
Er, no, I"m not. I'm pointing out that obsessive practice focussed on empowering oneself over others can turn people into monomanical and selfish individuals. If you want to be like Sagawa, then off you go and train like Sagawa, just don"t be surprised if you turn out like him.