I think you've identified the key issue: "What patent policy is in the best interests of society?" In granting monopoly rights to inventors, we incentivize innovation and society gets the benefit of new and useful inventions that might not have otherwise occurred as soon. On the other hand, society pays a higher price than it would if anyone were allowed to copy those inventions and distribute the benefits in the most economically efficient way.

There are various safeguards in the law that make this trade more appealing to the public. Since the monopoly rights are so strong (there is no "fair use", for example), there is a high level of scrutiny before granting them. And there are high standards that must be met ("subject-matter eligibility", "novelty", "non-obviousness", "definiteness", "enablement"). And even after a patent is granted, it is contestable (but, litigation is expensive). The big safeguard is that patents are limited to a term of 20 years (in the U.S.) and after that the invention is dedicated to the public.

I am pretty sure that the 20 year term is non-ideal. For some types of inventions (e.g., those requiring only a small investment of time or money to innovate) society may be granting inventors too long of a term. For other types of inventions, the term may be too short (e.g., where an innovation required a huge amount of investment). As you pointed out, the effects on society are hard to calculate.

Because the rules are "one size fits all", I am pretty sure we don't have an ideal system. However, I don't know if adding more rules would make the system better or worse. At least interested parties can petition for changes either through the courts or through the legislatures.