The original intent of the founders was to allow the creators of intellectual property to have an exclusive right to its earnings for a LIMITED time, after which that property was to enter the public domain where it could be used for derivative works that could themselves be copyrighted.
And I agree - the term is WAY too long.
One nit: The original term in the 1790 act was 28 years, or more accurately 14 yrs plus an optional 14 year extension. That seems reasonable enough. It really got ridiculous in the 1998 Act (nicknamed the Mickey Mouse Protection Act for obvious reasons).
Correct, it was to be like a patent. A limited term of exclusive (or licensed) use.
Now our nation’s cultural history is pwned by a few major corporations and the heirs see little if any return of their ancestors’ works.