The covenant is not just grunts or elites, but a lot of different aliens.. The reason I picked vex was because the vex, unlike the covenant, are timeless, and in being so are all but immortal. I see the argument. They could glass the planet and kill them all. Right now. They'd still have to go back into the past and kill them and the future and by the time they put a dent in... The covenant would be too thinned out. One Goblin against one Elite would end up in the Elite winning yes. Hell, maybe a "archon priest style" firefight might still end in the covenant winning. But they're organic. They age, they bleed, and they hesitate. The vex do not..