Ironically, the phrase I am trying to recall is just out of reach, so perhaps someone can help me with a phrase that describes a moving goal that is forever just out of reach.
I will try to provide some additional context on how I intend to use it. I am currently responsible for the rewrite of a large piece of software that needs to be supported in the interim. My experience in the industry tells me that if we were to release the new software only when it was a complete replacement for the old software, we would never actually release it. The correct approach is to release a functionally complete, but feature reduced product which can then be iterated upon.