The Framers authorized Congress to enact laws with respect to writings and discoveries; they did not command Congress to do so, however, nor did they… - David L. Lange

Yet in the end, the assertion that exclusivity equals productivity is essentially thin or testimonial or theoretical, or some combination of the three, and "believe" is the operative word. We do not know in absolute act whether intellectual property regimes significantly encourage intellectual productivity, much less whether they are "necessary".

Copyright claims to offer incentives to creative productivity, but in its commitment to exclusivity copyright also offers barriers to creativity, and often enough these more than offset such incentives as there are. [...] the harsh reality is that for most creative artists these industries offer neither prospects nor incentives. There are multiple and complex reasons for this reality, but among them is the inescapable fact that no one can create freely without building upon the creativity that has gone before.

Let us be direct: the dark side, and indeed a principal aim, of copyright is to suppress unauthorized expression for a period of time amounting, on average, to almost a century. This may or may not be an encouragement to originality or creativity or some other form of favored productivity in some; it is unquestionably repressive as to others.