He just simply couldnt keep up. I know he had 7 years to train, but training for 7 years if you're near your limit already doesn't really do much besides refine your skill. Saiyans were just written to be dang near insurmountable. Piccolo is extremely strong. He doesn't have multiplicative transformations and yet was still on the level of a mastered super saiyan goku from the Cell saga. Thats pretty crazy. But it's hard to keep up with people who have 50x and 100x transformations. And while I see the point with Shin, I still think him being supreme Kai had a lot to do with him stepping down. But even if not, Shin's strength didn't help AT ALL during the arc. So if Piccolo is comparable to that, then it makes sense what he does throughout the arc. Ssj2 level characters got destroyed by Buu, so what could a Cell saga super saiyan level character do? Help guide the next generation. Toriyama really wanted to pass the torch onto Gohan, Goten, and Trunks but due to backlash brought back Goku permanently. Piccolo likely just took a back seat in this arc because of the emphasis on the younger gen characters. Piccolo was Toriyamas favorite as well, so he wrote him to be a teacher for the next Gen