You say it's obvious?!?!?
... OK sure I can give you that. It was freakin obvious right from the start.
The reason it's the right choice is that it's the only choice with any real dramatic value (other than it really being Madara which was apparently not on the cards). Honestly tho ppl who were saying it HAD to be Shisui or HAD to be Izuna or HAD to be FREAKING KAGAMI?!?! These guys were complete and utter randoms and if it had been them everyone within the narutoverse (plus most ppl reading) would have been like "uuuuuh... whotf are you?"
It would have been completely stupid. Kagami? Really? Some random Uchiha from some random chapter? Other than Madara, I can't think of any other Uchiha that would have not seemed completely random and stupid. It HAD to be Obito, never mind all of the obvious clues like his jutsu which was CLEARLY the same as Kakashi's. Uchiha always go blind, just like their fanboys (OK sorry that was mean but it just came to me).
Is Naruto for kids? Yup. Is it obvious and simplistic? Totally. I really love the thing, but there's no need to pretend it is what it is not. Rant over, apologies in advance.