All it takes is one. Most (and again, this was not directed at you or this board; it was a general statement) thought they'd have to sacrifice Green, in addition to Splitter. I'm not trying to pat myself on the back, but I was one of the few who was steadfast that they'd find a way to keep Green. And it wasn't because I'm as knowledgeable about the cap as you or others, it was because when they want to keep people and they want to stay, they find a way to make it happen.
It might seem like semantics to you and others, but to Aldridge, it clearly matters whether he's the primary starter at center or not. They know this and I believe they have an understanding with him on this.
I wouldn't be surprised if West retires. In your scenario, where he doesn't and stays but Duncan does, I think they'd start Marjanovic over him. Duncan has started next to placeholder centers for a lot of his career, only for the so called third big to play as much or more and for Duncan to serve as the nominal center for the majority of his minutes. I think history would repeat itself.
Duncan and Ginobili are more than likely tethered together at this point and again, I think both will probably be back, which makes this whole thing moot.