It's not whether they can since we have known since TNG that a single person could technically run a starship if it were needed.It's the future. They probably don't need more than the bridge crew and a few sturdy engineers on call at any time. Especially now since they have autonomous robots doing the rough stuff.
The farther in the future you go, the less crew you likely need. I wouldn't be surprised if they have holographic crew as well, plus they have a super-smart hybrid AI capable of running the ship herself (although they might not know that yet).
Star Trek III showed us a Constitution with like six old guys running it on 90% automation. I always imagined that most of the Discovery crew were just scientists doing whatever. 90 is probably way more than they ever need.
It's that doing so would tank the efficiency of everything into the ground.
The thing they apparently forgot here is that it's not "90 people running the ship" it's "3 groups of 30 people" running the ship.