I’m sure you’re right for individual authors who are driven by a creative spark, but for, say, movies made by large studios, the length of copyright is directly tied to the value of the movie as an asset.
If that asset generates revenue for 120 years, then it’s slightly more valuable than an asset that generates revenue for 119 years, and considerably more valuable than an asset that generates revenue for 20 years.
The value of the asset is in turn directly linked to how much the studio is willing to pay for that asset. They will invest more money in a film they can milk for 120 years than one that goes public domain after 20.
Would studios be willing to invest $200m+ in movie projects if their revenue was curtailed by a shorter copyright term? I don’t know. Probably yes, if we were talking about 120->70. But 120->20? Maybe not.
A dramatic shortening of copyright terms is something of a referendum on whether we want big-budget IP to exist.
In a world of 20 year copyright, we would probably still have the LOTR books, but we probably wouldn’t have the LOTR movies.