Herrera was below average defensively the last couple of years then sat out two years as his speed declined. He is not good.
Quinn was new to the position and sometimes takes bad routes, but is also improving and has a good throwing arm. The team considers him better defensively than the other options other than Kingery where it is 50-50 on who plays CF and who plays a corner when both are in the game. It is the team's usage of Quinn and the eye test that he has improved defensively that is the test of whether he is better. The stats don't show it yet, but it is a small sample and much of that was several years ago when he was newer to the outfield.
Quinn's 648 OPS is not really "inflated" in your terms either. He has had only 131 at bats in the last 2 years. That is a small sample. He also has a 793 OPS in AA and 758 in AAA with good OBPs (358 and 349 OBP respectively). That is a large sample and shows he mastered those levels adequately (like a 4th OF would, not a starter unfortunately). He was better in AA than Moniak for instance and Moniak has not had the AAA experience yet.
Quinn is not great. But to interpret a tiny sample of a 27-year-old as a steady downward slope is just a bad interpretation of a small sample size in stats. He is younger than Herrera who actually had a read downward slope of performance in a big sample. Quinn needs a fair chance with modest expectations. That actually would have a better chance of success than Herrera who also comes with personal baggage and distractions.
Give him a chance. If it does not work in a few weeks go to plan B. Just stopping running Moniak out there. Jankowski at the very least is better in the present if you are going to do that.