Our government/society once placed constraints on people based on skin color, age, intelligence, faith, and sex. Past actions are not a validation of current or future actions.
The belief that IP rights were placed in the constitution as an incentive for further works is not tenuous. The main proponents at the time were businesses which were seeking to secure their profits, colorful language of the result not withstanding. Whether a large portion of the population agrees with such things, does not make it just.
I would assert that it is perfectly reasonable that one retain attribution to one's work, and allowing one to claim dominion over similar work is unreasonable. The first is an acknowledgement of a member's contribution to society, the second is detrimental to society, as this allows a single person who has benefited from society to hold the future of the rest of society hostage for their own purpose. Allowing one person to hold the future of others hostage is unreasonable and unfair. I have no problem with society continuing to reward contributing individuals, in any fashion, as long as it is not detrimental to others. Rich or poor, no one is more deserving than another, it is only the contribution that matters.
The idea that a company should be granted such rights, it is also unjust. Individuals are the source of creation, the company may be entitled to a majority of any reward/royalties but, attribution belongs to those who did the work. To pretend that an employer is the source of such effort is fraudulent. Drawing a line between a creator and a worker would still have to be drawn.
Whether such rewards for creation be lifetime or limited is debatable based on the value to society. Those who cure and those who entertain, fall on different ends of the scale.
If you create a cure for cancer, I would not deny you your reward, in fact such a thing may warrant some form of monetary reward for your whole life. I would deny you dominion to decide when/who makes it, and therefore who might receive it. If you create wonderful novel, you should be rewarded for contribution, and to a small part that of any derivative works. How such things could be setup and fairly measured and rewarded is a much harder question, than the fairness of what we have today, companies raping artists for all they're worth, with a few outliers.
The current system is an injustice but, so many things are. There is no perfect solution but, when problems in the current system are ignored so that a few may benefit, that is what causes people to seek redress. Perceived injustice will always drive people to right such things in a manner they feel acceptable. Dismissing those wishing for more a more just system as acting in their own self-interest ignores those who have benefited from acting in the own self interest in an unjust system. All are acting in self-interest but, the scales are not even.