The plan was never to kill Sukuna

It was always to save Megumi. Both Yuta and Maki had killshots on Sukuna they could have taken but they chose options they knew Sukuna could survive because SAVING MEGUMI was always the main objective. Sukuna was dead in the domain easily if just killing him the plan. What’s stopping Yuta from doing to Sukuna’s head what he did to his arm in the first panel?