A Friend of mine has long noted that when the Nation was founded, both Patents and Copyrights were the same length. (21 years, I believe.) Over the years, Copyrights have gotten more protected, and patents less so.
My friend also points out that Patents are for things that actually improve the world, that actually make it fundamentally better than it was. Music, Books, Art, etc. do not produce any tangible benefit to the world. They don't clean water, they don't feed people, they don't shelter anyone, and yet they are better protected than something useful, something that actually makes a difference to humanity.
Whatever arguments you have put forth in support of nearly perpetual copyrights, I would say they should also apply to patents. If anything, Patents ought to be even more heavily rewarded than copyrights, because patents produce tangible improvements. Sometimes forever.
Do you think it would be reasonable to keep a patent for 70 years after the death of the inventor, as is done currently with copyrights?
Yes. . . but I would add a couple more thing to my list.
With those provisos, I would. . . but drop the 70 years. Why put an arbitrary limit on it. . . and on copyrights.
What gives the government the right to seize intellectual property without due process any more than personal or real property? This way, death or life is removed from the equation. What about inventors or authors who sell their IP to a corporation? Such a corporation does not die. My solution obviates that problem.
I would not be opposed to handling copyrights in the same manner. Have a declining value in licensing after a period of exclusive use. Problems solved. Public fair use of copyright loosened a bit. Digital reading of a book or viewing of movies after exclusive use on the FRAND basis. . . and declining basis thereafter. For photographs, books, movies, and music, it might be necessary to do a stepped FRAND rate, quantum, so to speak.