I agree - though Seamus is our first choice and imo you don't lose your place due to injury.
Holgate is a centre-half who will be a full-back, but he made mistakes and still has a lot to learn.
I hope he gets more gametime. Same with Davies.
But I think it's pretty clear Koeman wanted to look at the experienced first-teamers and what they had to offer first. Which is a completely logical thing to do.
Now, if Davies comes into the team, he'll be hyped by our fans so much (and don't even argue he wouldn't be) that he couldn't possibly live up to expectations when he inevitably comes in and we get beat 3-0 or something.
It's a lot easier to bring youngsters into a team playing with confidence.
That being said, I do think he should be more involved, but that's based on a few U23 games I've seen and his minutes on the pitch for the first team. I don't see him day in day out.