I guess the explanation to why someone deserves to be paid for something as long as they live is: if somebody is creating something to be sold to other people, and that is their job, why should it matter when that thing is sold to someone? A policeman isn't being paid for keeping someone in Jail, they are being paid to put them there. Content creators don't get paid for creating a work, they get paid for selling it to somebody- so why an arbitrary restriction saying once X years are up, you can't sell it anymore (or can't effectively since anyone can have it for free).

The answer is, it is in the common interests to have these rights expire at some point, since after a certain period of time the utility of having everyone be able to do what they want with the work is greater than the expected value the content creator would be able to extract. But if you believe that copyright should be valid for any period of time at all, don't act like it is a given that it should expire after some arbitrary period of time. You are taking away a right from somebody in the interests of the public good, and there is always going to be discussion when that happens because you are taking away from one group of people specifically to benefit society as a whole.