Promoting the Progress
The purpose of the IP clause of the US constitution was not to encourage innovation. The idea was not derived from a fear that, without sufficient mechanisms to compensate innovators and creators, that they would simply choose not to bother. Such illusory boogeymen are pure fabrications made up by information-age mafioso.
No, the point was not to promote innovation, but to promote progress - specifically, "the progress of science and the useful arts". This, by the way, is the only place in the entire constitution where the word "progress" is used explicitly.
As we conceive of them, we do not understand science and the arts as being things done privately. Academics have a well-known phrase: "publish or perish". By this, they mean that scientific work not made available to the public (thus the term "publish") does no good for human scientific knowledge , to say nothing of the scientist's academic reputation. The same is true of the arts.
It should be noted that, even in the 18th century, innovation was clearly happening in many areas. It's also pretty clear that innovation and creation are usually their own rewards, regardless of the policy used to regulate their dissemination. The concern that motivated the framers was not that inventors and creators weren't innovating or creating, but that they would fail to share their works, and, as a result, that they would be lost rather than added to the historical pool of human knowledge.
The IP clause of the constitution was added to benefit, not innovators and creators, but the public domain. Patents and copyrights were designed to create a temporary (and otherwise non-existent!) incentive, not to innovate or create in themselves, but to publish and share intellectual works for the benefit of everyone, and for posterity. Otherwise, inventors and creators might think themselves better served by keeping their secrets to themselves entirely. The IP clause was created as a compromise to entice creative thought into the open with a carrot just big enough to accomplish this goal, and no larger, so that human knowledge could be enriched for all of history following.
With modern industrial giants claiming this as their entitlement alone, and twisting it for their own purposes, they have managed to distort its original intent in such a way as to convince us that its purpose was specifically to enrich them. On the contrary, the IP clause was intended to coerce disingenuous actors just like them into giving up control over their hoarded knowledge in order to enrich everyone. We would do well to bear this in mind as we endeavor to reform our IP policy, and recover it from the distortions of the industrial copyright cartel.