In this instance, it doesn't really matter what you feel because you are acknowledging the war would have been significantly shorter, and choosing to disregard canon you clearly understand. Here's yet another instance, in Halo Wars, if Red Team wasn't around, the Covenant would have overrun the team sent to destabilize the artificial sun, and the Covenant would have gained control of a fleet of Forerunner warships, ending the war in only a few years.