Character, virtue, and what I would call spirituality are typically behind why good prevails against evil in MCU. From great power requiring great responsibility, to what Phil Coulson calls conviction, there is always what I’d call morality that allows the heroes to prevail against a more powerful foe. For Iron Man, its justice v. profit. For Thor, sacrifice v. ego. Capt. America, Freedom v. control; Hulk, trust v. ruthlessness. GoG, friendship v. revenge.
I don’t know what the Dr. Strange movie will do relative to power sources and if they can avoid everything reducing to infinity stones. The occult aspect will be offputting for some people, sure, but Harry Potter seemed to do okay without that consituency. But if there is a unifying principle to why power works this way in MCU, I think it is that we can only really use power to serve others. When we think we use it to serve the self, the power uses us.
Responding to this post: http://www.looper.com/4166/doctor-strange-cause-big-problems-marvel/