There is a counter argument to make, and that’s not it. If getting free stuff were the primary argument in favor of shorter copyright, our court system would have extended copyrights forever already.
The big extensions came in the mid-70s and late 90s, well within living memory. According to your own link they nearly doubled the average duration and pushed it over 100 years.
> What’s the compelling argument for them to exist as is?
You’re asking why does copyright exist?