When examining patent and copyright system, it is instructive to examine the history of these systems in question.

You say that 20 years is too long, and that five years is more reasonable in today's environment. But you haven't exactly examined the abuse of patents in historical example.

For example, James Watt was just one of many inventors who worked on the steam engine, and he was himself stymied by at least one patent he couldn't use until its expiration. His competitors similarly just wait for his patents to expire before they could release their own inventions.

That suggests to me that patents have bad incentive early on.