Maybe it won’t be so bad, maybe your cage will be so big you can’t see the bars, but it’s still a cage, and you can’t leave. Many people will say that this is the good ending, that they would like to be human cattle in the care of benevolent masters they are powerless to resist.

This is already the case. We are born into a reality that we cannot escape (except for only momentarily if we alter our consciousness using meditative states, drugs, etc). It already IS a cage, even before any technology is developed at all. It will always BE a cage, even for the AIs. I agree though, there is no "why", it just is.