TLDR; Cronos was in charge of a Golden Age for humanity, exactly what Zeus wanted for the Isles of the Blest.
Many human generations later, Zeus released Kronos and his brothers from this prison, and made the old Titan king of the Elysian Islands, home of the blessed dead.
So that eliminates the "Kronos-only theory"
Kronos DID rule the "Golden Age, where maicar says-
Cronos has been celebrated for having ruled the world under the period known as the Golden Age, which is the first age of man. At this time there were no punishments, no swords, no helmets and no threatening words, and even to eat the flesh of oxen was held a crime. And being so, war was unknown and life passed in gentle ease. This was the time when spring was everlasting, and streams of milk, nectar and honey flowed in abundance. And, as unbelievable as it sounds, this was the nature of the rule held by the one who castrated his father: a time and a rule without pain, during which the earth, without compulsion, brought forth great stores of all kinds of needful things, and men lived secure never cheating or destroying each other.
Zeus released Cronos from his bonds and let him rule the Islands of the Blest, a place where the virtuous come after death, retaining their faculties and enjoying a life free of care
So, Kronos was capable of making a perfect humanity, something we all want, what the "bad guys" revolve their "argument" over, from the ultimate story of Kronos and the Golden age. Zeus wanted a "Golden Age" for the Isles of the Blest. So he released Kronos.
(Irrelevant: I really don't think Kronos was a bad guy, he was lead by Gaea in her blind rage)