the Covenant is not destroyed but just too weak to wage war against the UNSC as most of their forces
got disbanded after the revolt of the Elites where also some Grunts, Jackals and even Hunters took part.
With all high ranked prophets killed.
But there are still many Brutes, Grunts, Jackals, Hunters, Skirmishers in the galaxy who are still hostile
to mankind and Sangheilis/Elites.