During the grunt rebellion the covenant decided to glass the unggoy homeworld. If they didn’t glass the unggoy homeworld would the grunts have won?
Probably not. As hardy or whatever as Grunts may seem, the Elites would have eventually put them down. Grunts are submissive and the rebellion was just an outburst. Compare that to humanity which constantly put up a fight (and sometimes won) no matter how lopsided a battle seemed.
Glassing Balaho was a retaliatory measure by the Covenant to stop the rebellion short, not a last ditch effort to bring them back into the fold. I don’t see them winning in a straight up fight, even if Balaho wasn’t glassed.