I didn't say copyright only existed for 100 years. I said the "system", as you eluded to, has only existed for the last 100 years.

It's a lot less than 100 years, actually. I put it at the 1976 copyright extension act where things went totally off the rails.

(You could argue the 1909 act, which was accompanied by discussion in the Senate re: benefiting copyright holders in their old age and providing for their descendants was where things went wrong. The term didn't become outrageous at that time though. The 1909 act also required registration of copyrights as a balancing feature-- a requirement that would be removed subsequently.)

re: terms and the lobby

The original term of copyright in the US (1790 copyright act) was 14 years w/ an optional single renewal to 28 years (drawing inspiration from the Statute of Anne).

The 1831 copyright act increased the term to 28 years with an optional 14 year renewal. Noah Webster, cited by some as the first copyright lobbyist in the US, had an influence on this act (as did European copyright terms and "normalization" of US copyrights to match Europe-- a theme that repeats).

Samuel Clemens suggested copyright terms should be perpetual in an appearance in 1906 before the British House of Lords. Presumably he was voicing the same opinion in the US. He wrote about how any "reasonable author" would like copyright to "take care of his children" and "let the grand-children take care of themselves". (Why the children of copyright holders should be entitled to a "free ride" while others' children are not is a topic worthy of debate. Being the child of a copyright holder doesn't confer some inability to make one's own way in the world, to my mind.)

In 1909 the term of the optional renewal was extended to 28 years.

Up to and including 1909 US copyright seems fairly reasonable to me. It certainly seems capable of providing incentive to creators. A 56 year term is longer than I'd like, personally, but it isn't ridiculous. The public domain is still enriched and creators have the opportunity to create new derivative works.

In 1976 the term was extended again to either 75 years (works for hire) or life of the author plus 50 years. This is where, in my mind, the "social contract" became radically and unfairly stilted in favor of copyright holders (not necessarily creators-- since work-for-hire was codified in 1909 and became a normalized practice between '09 and '76). Corporate interests lobbied hard for the '76 changes and again for the Sonny Bono extension act in 1998 (Disney figuring in both times, for example).

I regard the '76 copyright extension as nothing other than a "pull up the ladder" action, and the '98 extension as even more so. So much of human creation is derivative. The '76 extension robbed significantly from the public domain. The '98 act changed the rules retroactively again, pushing the ability of creators to make new derivative works out to ridiculous future dates.

To my mind everything after the 1909 act has been serving the interests of copyright holders w/o regard for balancing the good of society.

Society has a shitty lobby (and post Citizens United money equals speech). It's never going back to being a fair system.

re: doing well for one's offspring - Consolidation of wealth and generational wealth are concepts worthy of debate. I don't think they're an unalloyed good, nor do I think I'm alone in that thinking.