This is all going to be from the Paragon perspective since Renegade Control does result in what you're proposing, Matt:
You're right that in taking on Control, Shepard is taking on an
immense amount of power onto his shoulders but Shepard continually makes those decisions throughout the game.
He decides the future of the Krogan,
he decides whether the Geth die, the Quarians die or neither. Whether he intended to or not Shepard is already shaping the fate of the galaxy.
There's something Coats (the british sniper) says to Shepard that I think ties nicely into all of this when Shepard expresses the belief that he's still just a soldier and Coats tells him that while he may see himself that way he is
not.
There was a line in the new Avengers flick where Tony Stark expresses the idea that power is a terrible privilege - that's kind of how I see Paragon Shep's relationship to the Reapers, that by being here, being the one who made his way to this moment and to this decision he has taken it upon himself to wield that privilege and more importantly wield it
responsibly, so that as the new Shepard AI states "every race can have a voice in its own future," to not turn into the tyrant that Illusive Man or Renegade Shepard would have become.
I think Control is the ending where Shepard trusts in the strength of
who he is to win the day.
Or disappearing back into Dark Space, or whatever they called it, only to reappear when **** hits the fan and to tell everyone to cool the hell down.
I think there's enough room for you to imagine that this is what Shepard does with the Reapers. Obviously they'll be around during the reconstruction phase but there's nothing to say that afterwards he doesn't retreat from galactic space to return only in cases of cataclysm or disaster.
I see. The endings still suck. 
