I'm agnostic on whether I personally think Manning should have replaced Ewers. I can see reasonable arguments for and against. What I am not agnostic on is whether or not I think Sark is in a better position to make that decision than I am (or anyone else on this thread). He knows more about football than we do, he knows more about the players than we do, he knows more about the locker room than we do. He's not under any meaningful outside pressure to make a decision that is detrimental to the team as a whole. Sark isn't perfect, but I think he's at least earned a little trust in how he manages his roster and his team culture.
We can speculate all day about why Sark stuck with Ewers, but we won't know. It could be misplaced loyalty or concern about Sark's image as a QB developer, and I can't definitely say it isn't, but it could be a million other real and substantive concerns indicating that Ewers was the better choice. And there's no counterfactual for us to use in order to prove the issue one way or the other - we can't go back in time, have Sark start Manning, then compare how the season went.
To be clear, I'm happy to speculate and I'm happy to talk about Ewers' level of play. But the conversation loses all value when you become dug in to a position and are unwilling to keep in mind what I've just said - we can't know one way or the other what the right choice was, and that reasonable people can have different opinions. Having a different opinion doesn't make the other person malicious, arbitrary, or stupid.
For my bit of speculation, I also am surprised at how few people have mentioned a very basic reason that Sark may have gone with Ewers - coaches play the players they trust, and people tend to trust someone they know well. In other words, not only is Ewers more experienced than Manning, but Sark is has more experience with Ewers. As I've noted several times, Ewers has his flaws but Sark knows those flaws. He can coach around them. That's not just a Sark thing, that's most coaches. Saban going to Tua over Hurts is so remarkable because it was so unusual. I'm just never surprised when a coach goes with a veteran. There usually has to be a compelling reason to switch to the younger guy.