My guess is that AA would assume that your main purpose is to get where you wanted to go when you wanted to get there. Certainly leaving you on the same flight (since it exists) makes more sense than rerouting just for COS. (And I mean that it makes more sense for normal people, if not for us, and their system is geared for "normals" since there are more of them). If the flight you were on had gone away, I would venture to say that they would have tried to find a suitable alternative with First available.
Cheers.
That certainly makes sense in the context of a paid ticket, but when one is using miles (meaning it is more than likely leisure as opposed to business travel), I would think that more people would want to get what they paid for, especially if a refund would not be forthcoming (or, at best, be easy to calculate).
In other words, if I was traveling for business, I would probably prefer the F to J downgrade (but would probably not prefer a J to Y downgrade), but if I am traveling for pleasure and have paid for a premium travel experience, that experience is what I want. Not sure if I am explaining this well.