Why doesn't Shadow just stay dead then? It would have been a lot more fantastic if he did instead of being revived every game.
Easy reason? Because Sega saw the fanboy-appealing potential that Shadow had, and wanted to utilize him some more.
Never mind the fact that friggin METAL SONIC was an already established character, who not only served Shadow's basic purpose years before hand, and could have easily been written to do more in that vein.
To me, Shadow was a fairly redundant character from the outset. Even the whole "Ultimate Lifeform" angle was something that Metal Sonic was already doing years before hand.
Zero should have stayed dead back in X1 or at X5/X6 like he was supposed to but he revived yet again and he he even had his own series.
Zero is different, in the notion that, by all sake and reason, he was closer to the "original concept" that Inafune wanted for a "Rockman X". But Capcom wasn't going to play ball by his rules. So, as a consolation prize, he shoe-horned Zero in there any way, and was willing to just play the waiting game.