The existence of Silver makes more sense if you think of it in this way. Basically, when they created Sonic, they struck lighting in a bottle. Everything about that from the design, aesthetics, how it ties into the gameplay, is pretty much perfect and thus became a big franchise. Within ten years they managed to create yet another character that's just as cool, just as marketable, and nearly as popular as Sonic himself with Shadow. It was like striking lighting in a bottle twice. After this they must have thought well if they could do it twice, they could do it a third time. Since Sonic is Gokuu, and Shadow is Vegeta, with basically the entire staff being fans of Dragon Ball then logically the next one must be based on the next coolest character Trunks. Managing to get another character on the level of Sonic and Shadow is difficult but like I said they must have thought it would be easier to work out since they already did it two times before. It did not help that 06 is his debut game. If that game turned out better than maybe it would have worked a bit better.