I'm not sure why my spouse, children or grandchildren etc, shouldn't be allowed to continue to reap the rewards of my work after I'm gone.
And they can do so. IP is intangible property, just like anything else. If you died five seconds after your work is published, it can and should be included with your other assets and liabilities in your estate as guaranteed by your Constitutional right to property and due process.
However, an unlimited time length of govt sanctioned and enforced monopoly on IP (in human lifespan terms of unlimited, obviously) is directly contrary to the specific intent enumerated in the Constitution, as it would be intentional usage of government force in order to inhibit the progress of science and useful arts. Ergo, a direct violation of the Constitution, thus illegal.
If I may put it in more material terms. If you make a physical object. A book or statue. It is your's forever. You or your estate can keep it as long as you'd like, subject to taxation and whatnot. No one is disputing that part.
We are merely talking about the right to make copies of said form or content. If I make a copy of said book or statue, I have not physically deprived you or your estate of your physical property. Ergo, not theft. I can only infringe upon your intellectual property, which is still potentially a crime. Now, in the interest of promoting the sciences and useful arts, the government is accorded the right to use force to stop me from making copies and thus infringing on your IP for a limited time. So you can make some money off licensing or whatnot. The government is NOT accorded the right to use force against me for all time, as that would be hostile to the progress of science and the useful arts.
If folks want to pass copyright laws that inhibit the progress of science and the useful arts, that is fine. But they have to amend the Constitution first. Otherwise, no go.