Look, what would Piccolo accomplish going in the ROSAT after witnessing Trunks and Cell's power? Whenever Piccolo trained and gained power, we all knew that he surpassed the previous strongest. For example,
Piccolo trained for 6 months to surpass Raditz and he did.
Piccolo trained in King Kai's planet and the last time he fought someone stronger than him was Nappa and he surpassed him.
Piccolo trained to surpass Trunks in order to fight the Androids and he did.
Piccolo knows his chances better than everyone and if he thinks that he can surpass Cell and Trunks by training in the ROSAT then sure he can. Otherwise, he would just wait there and hoped for Goku to get stronger to do the job done just like he did on Boo arc where he knew that he's useless against Boo and placed his hopes to the kids.