[copyleft-next] Isn't the copyleft sunset a disadvantage to free software

To quote section 8 of copyleft-next-0.3.1: ` 8. Copyleft Sunset The conditions in sections 3 through 5 no longer apply once fifteen years have elapsed from the date of My first Distribution of My Work under this License. ' I feel that addressing copyright obsolescence should be follow whatever the copyright law says - which is currently 120 years or the author's lifetime plus 70 years - because the issue I am seeing with the "Copyleft Sunset" clause is that it puts free software as a disadvantage in comparison to non-free software. As a free software maximalist I want all software in the world to never be proprietary including the old software that I wrote ages ago and nobody uses anymore, I want that too to remain free forever and after.