I don't think Buck's decision was just about this particular game. I think everybody knew Matusz was huffin' and puffin'... and that certainly includes Showalter.Buck shoud have pinch hit for him. He then shouldn't have let him throw 30 pitches in the 6th inning, in a game which he labored. Yes, the onus is on BMat to get the job done but it's on Buck to make sure he is in a position to succeed....they both failed.
My guess (based on not much) is that Buck thinks it's time for Matusz to grow up and act like somebody. I think this was a case of Buck saying to him, "Yeah, I know you're tired... so what? It's still your game... whatcha gonna do?"