This is the big conceit of copyleft licenses: that "derivative works" are exclusively "modified and extended versions of the program" to use the FSF's terminology, and not a small piece of a much larger system.
It's easy to imagine wanting to add Pijul support to Tower, SourceTree, Gerrit, Phabricator, etc. - projects that all dwarf Pijul. But these projects will be unable or unwilling to risk doing so, because of the AGPL.
