Is Predictability Impossible?  

In general, no. There are some software developments where predictability is possible. Organizations such as NASA's space shuttle software group are a prime example of where software development can be predicted. It requires a lot of ceremony, plenty of time, a large team, and stable requirements. There are projects out there that are space shuttles. However I don't think much business software fits into that category. For this you need a different kind of process. One of the big dangers is to pretend that you can follow a predictable process when you can't. People who work on methodology are not very good at identifying boundary conditions.

However letting to go predictability does't mean you have to revert to uncontrollable chaos. Instead you need a process that can give you control over an unpredictability. That's what adaptivity is all that.