You're assuming that healthy Arch > injured Quinn. You may be right, but we really aren't in a position to know.
I hope that Sark wouldn't allow his loyalty to Ewers or pride in his ability to develop QBs to lead to a decision that negatively affects the team. And we really haven't seen anything in Sark's conduct that suggest we should assume that's the case. He is maybe a little too loyal, but he's not loyal to a fault. He pulled Ewers and brought Arch in against UGA in Austin - turns out UGA was just going to kick our ass regardless of who was under center. You also can't discount the limited ceiling of our run game and the impact it had on the rest of our offense. Passing game can't afford to be simple / predictable if teams know you're not going to run. A more experienced QB can open up the playbook.
To be fair, I'm making a lot of assumptions myself. But I'm willing to give Sark the benefit of the doubt here - even injured, Ewer's experience probably gives him the edge over Arch.