You could make an argument that Pyne would have been a sitting duck against South Carolina, and the pass game wouldn't have been there had he played, as South Carolina wouldn't have respected him as a run threat, and would have dropped 8 and made Pyne look like a deer in the headlights.
Buchner made 2-3 HUGE plays with his legs that Pyne couldn't have made, as well as a deep throw to Lenzy that Pyne likely doesn't get off, and they helped win the game. He offset two bad mistakes with 4+ great plays. Pyne is a net neutral, generally, and would have likely been the same in the bowl game. A net neutral loses the bowl game, imo.
People hate hearing it, but Buchner needs more seasoning, simple as that. Keeping him healthy to GET that experience is a different debate/issue/argument entirely.