I would add only one thing to this;Chimera30 wrote:Before crossing the portal, humanity didn't know what to expect about the Ark. Johnson knew that the Ark could remotely activate all the Halos from Guilty Spark, so it's likely High Command knew about that as well. But Chief and the Elites went to the Ark to look for Cortana's "solution" to the Covenant and the Flood (and chase after Truth to kill him), while Lord Hood wanted everyone to stay on Earth and defend it from the impending Flood attack. So the UNSC wasn't really in a rush to go after him; specifically it was Chief and the Elites, and some of the UNSC forces just decided to accompany them out of loyalty (and I guess Lord Hood was like "eh, screw it" and let them go).
The only thing worse than having an Ark with an unknown location AND a Prophet on it is having both of those and a closed portal to get there. Once that portal closes Truth has all the time he could want to find a way to light it, even if it did require a human touch. IIRC there would have been human specimens on the Ark that would only have needed to be reactivated in order to fire the rings. Find another corrupted Monitor to start that process and the conclusion is the Great Journey.