My understanding is that the copyleft sunset was added as some sort of
commentary on the seemingly never-ending length of copyright.
But at the same time all of those retroactive and future time
extensions that have happened (at least in the U.S.) only make copyleft
stronger, IMHO. Having pushed for strong copyleft adoption it feels
strange to systematically adopt a method to end it in a rote automatic
way without further consideration of the situation for a particular
work. The copyleft sunset is one of my reasons for not using
copyleft-next and why I continue to use the AGPLv3-or-later, which
would tie the end of the copyleft to the end of the copyright. That,
and the lack of a network services clause.