Well…In your post, you seem to perceive this issue only from the monetary point of view : the author made enough money with his work, so, now, everybody should be able to publish, play, etc…it and if he wants more money he should write another book, piece of music, etc…
But there’s also the issue of the moral property. For instance, you could write a book, and 20 years later i could make, say a pornographic movie loosely based on it but expecting to cash on its famous title. And there’s nothing you could say or do about it. I think that’s a significant issue.
Also, technical improvments are somewhat needed. If you sit on you patent your idea and sit on it for 70+ years, or even if you only strongly restrict its use, it’s impairing technical progress. Especially since, most probably, someone else would have come up sooner or later with the same idea. If yiou didn’t invent the phone, 2 years, 5 years or 10 years later someone would have invented it. The possibility to make a phone was preexisting. You just was the first to figure out how to make it work. But I don’t think you have an absolute right on the concept of telephone. Nor the right to, for instance, prevent everybody from using a telephone during 70 years, or massively restricting its use because you’re the only one selling telephones, and only at an incredible price.
On the other hand, an artistic work is completely original. If you write a symphony, nobody else would have written the same symphony, ever. You didn’t merely figured out something preexisting, you fully created it. So, it’s fully yours. And preventing people from playing your symphony won’t be an impairment for other musicians. They’ll write other symphonies, and even if you sit on yours for 70+ years, nobody will be annoyed.
To sum up (I’m not sure i was really clear), there are two main differences between an artistic creation and a technical innovation :
1)The artistic creation totally belongs to you since without you it would have never existed. The technical invention is only something you were the first to figure out. But it already potentially existed, and without you it would have been eventually created. So your rights on this idea are IMO more limited.
2)The artistic creation isn’t necessary. The world will goes on even if nobody ever read your book. The technical innovation is much more needed. By limiting access to it, you’re limiting further progress. So, it would be very inconveniencing to have 70+ years patents, while having 70+ years copyrights isn’t an issue at all.