We might have saner copyright length with all kinds of butterflies with regards to culture and digital rights. Less generic pop culture, more real culture and self-expression, more liberal societies overall.
While Disney was often cited over the changes in copyright law in 1978 and 1998, the entertainment industry overall would have pushed the changes with or without Disney.
From 1909 to 1978, copyrights had a maximum term of 56 years. That means any work copyrighted in 1920 would become public domain after 1976. The entertainment industry as a whole fought to keep early cinema and recorded music under copyright and the result was a sweeping change in 1978 that changed the term to 75 years for collaborative work (life plus 55 years for individual work). In 1998, Congressman Sonny Bono pushed a law that added another 20 years to the term, so a 1930 film is protected until 2025.
Now, the Disney characters have yet another form of protection, even if the copyrights run out: they are trademarks. Unlike copyrights and patents, trademarks can be renewed indefinitely.
I do agree the 1978 law imposes sweeping restrictions that stifles creativity. Prior to that year, works had to be published with a notice, usually the circled letter (c) and the year; and the copyright had to be filed (with fee) that year. Quotes of non-copyrighted work could be more open.
Today, a copyright is created the moment a sound, picture or sentence is written, recorded, typed in e-mail or otherwise "fixed in tangible form." I am not sure why the entertainment industry would find such restrictions to their advantage, but here we are.