The grunts arnt bad, but its true it doesnt follow canon. But im sure 343 will say something like the covenants bio-engineered to make them more efficient during combat. It could be viable, since shifting their center of gravity, skin, and toe placement could affect physical attributes.
MC looks great, though.
Changing canon after such a mistake that could’ve easily been fixed with just a small amount of remodeling is a really bad thing to do. I mean, bio-engineering really sounds more like a self defending excuse than a valid plot twist. Besides, making a creature with an exoskeleton to a creature with an endoskeleton really doesn’t work like that. Besides, in combat, you actually benefit from a hard exoskeleton as that means you don’t need as much armor.
So, that wouldn’t really be a good excuse for the look of the Grunts. Nothing would. The best thing for them to do would be to change to look back to be more grunty.