I see no upside to Sanchez's move to the pen. I think we are being overly protective against something that has no indication of happening, and it may cost us majorly right now.
I don't have the necessary information to come up with a counter to your point here. But I find it extremely difficult to believe that they would limit Aaron's innings if injuries were essentially a free for all. We've heard about the rule of thumb 30% increase for a long time.
You say there's no upside and that may be the case because there's no way of knowing for sure what will happen for the decision they don't make(keeping him in the rotation probably), but there's potentially an upside there that includes avoiding injury risk that makes him a long term better pitcher, and gives us a healthy Sanchez for the next 2 or 3 years.
That could have happened anyway even if they leave him there, so that's where they have to weigh the risk v. reward and I don't have the proper info to make such a decision.
Just for example it could be a Sanchez in the rotation + 40% chance he gets hurt in the next 2 years v. Sanchez in the pen + 15% chance he gets hurt in the next 2 years.
Like I said I don't know any specifics and I doubt anyone here does so I don't see the point in arguing about the Sanchez decision specifically, I'd just assume that since they're doing it, there's some risk avoidance here and the decision is whether that future risk avoidance is worth the loss of him pitching right now. And that's not a decision I'm informed enough to make. So I just accept what happens and move on because I don't know the right answer