All of your points in favor of debian's system really only apply if only use Debian's official package repositories. Something that you can absolutely do in Urbit as well. Nothing says you have to pull packages from every possible location out there. Urbit can absolutely have it's share of official repositories of applications with the same quality and safety guarantees that Debian has. And indeed many of the apps you get already come from a default official source. The star or galaxy you got your ship from.