Copyright statutes benefit society by providing incentive to produce original works, so some kind of copyright statute is ultimately justified.
Conclusion 1 (so)
Some kind of copyright statute is justified.
Evidence
Copyright statutes benefit society by providing incentive to produce original works.
Conclusion 2
Granting copyright protection for the life of the author plus several decades is too long.
Evidence (since)
The societal benefit from additional years of copyright is more than offset by the society cost.
Evaluate
This is a weird argument because there are two conclusions here, but we would probably assume that the first one was background and that our job is really to support the one in the final sentence.
If we were just spelling out the missing link, it would sound like this:
Societal benefit of Too long a several decades after → period to death is more than grant protection offset by societal cost
Goal
Look for an answer that talks about societal cost outweighing societal benefit as a way to argue that giving copyright protection for decades after an author dies is too long / too much.