I dunno if Wrawrinka and Cilic are capable of having good enough tournaments then I see no reason why Murray isn't capable of doing the same. We're assuming Djokavics current ridiculous form will continue too, he may well have a dip in form that other players can take advantage of, or injury. I don't think it's at all unreasonable to suggest another 2 majors are within Murrays reach.
The thing with Djokovic is he has been consistently outstanding every year. Even when he doesn't win it all, he is still usually beaten by eventual winners playing their best ever tournaments. I also don't think Murrays game has enough aggression and is too passive to have tournaments like Wawrinka and Cilic have pulled off. Not saying he is worse then him because he is consistently a level above them both but those two (Along with a couple others) have reached tournament levels Murray hasn't. I also don't think he ever will reach that level needed to beat Djokovic.
The thing going in Murrays favour is the lack of talent younger then him. The new generation are crap. However, on the flip side, he is a week older then Djokovic who has an equally brilliant physique and fitness, if not better. Its hard looking past the Serbian tosser TBH.