Well... actuely The Forerunners had taken up a role as custodians of the galaxy and believed that this \"Mantle of Responsibility\" had been passed down to them by the Precursors, (an even more powerful race that preceded the Forerunner civilization.) they believed that they held an impermanent place in the universe\'s Living Time and that they would be succeeded by other, better races. Before their disappearance from the galaxy, the Forerunners would name humanity their successors, identifying them with the title \"Reclaimer\".
on a sidenote, the didect \"hated\" humanity due to the fact that before the rings were activated the (at this point humans were FAR more advansed then they are in the halo game series) took up the job of \"custodians of the Mantle\" by chaceing down the Flood which ended up on a few forerunner planets which the humans then destroyed without notice to the Forerunners which they inturn took it as a sign of war which was later founf out that the humans jsut couldnt take the chances of allowing the flood to spread. the didect never forgave the humans but the librerian (his wife) understood the humans reasoning thus recreated humanity starting at a more \"tribal\" stage in evolution which the didect turned into his own army, turnign them into materials used to engineer \"promethians\"
I could go on though I think that explaisn that Forerunners were NOT infact the best. both the humans and the Precursors surpased them.