It lets you end things on your own terms, or at least on terms that you acknowledge. It's a chance to memorialize your alliance, share some memories, and transition the community's infrastructure (forums, Discord, IRC channels) into a gathering place for former members who want to come back and walk down memory lane every now and then.
Other alliances have gone out of their way to celebrate peace. NPO and its allies touted it as a sign of their success in years past, for instance.
And GPA was always an advocate of deliberate neutrality, not apathy or inactivity. People who create a nation and never log back in are model community members by the standard you outlined here.
My advice is to disband. If GPA has no desire to play CN anymore, there's no point in maintaining a formal, comatose presence. Anyone who cares about their nation would do better in a more active alliance; anyone who doesn't could still visit GPA's forums or Discord.