There’s no counter-argument to make beyond my self-interest in consuming creative works at very low cost and with very few limitations outweighing your self-interest in profiting from your works.
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?