But in most cases, it's
not considerably easier to make money, at least significant amounts of it, several decades after publishing a creative work. Who is paying the same price for software from 20 or 30 years ago that they pay for the equivalent today, or even 10% as much? Most big movies make the lion's share of their revenue during the initial phase of distribution in theatres and then the second phase of availability via streaming and physical media over the next few years. A lot of big games make a large chunk of their money in just the first few weeks after launch. Traditionally, a book that wasn't still making a lot of money would go out of print, which meant that for anything but the classics or best-selling authors, you'd have to look for a bookshop that still had a copy on the shelves or some second-hand source after a few years.
If copyright is primarily an economic tool, intended to create market forces similar to other products and services in order to promote activity within the creative sector, then a much shorter copyright period is perfectly justifiable. The few works that still make significant money after several decades usually also made huge money in the decades before, so copyright would still provide a strong incentive to create and share such works.
One interesting thing about modern economic models supported by copyright is that they do make more of the long tail viable: a work aimed at a relatively small niche or with unusually high production costs can still be economic to produce if you can recoup the costs over a longer period, and with digital storage and distribution this becomes more practical. But even then, how many works would be viable after 40 or 50 or 100 years but not after 20 or 30?