Signing both Mathieu and Vermaelen was Bartra's end. There's no football team in the world that can give five centre backs enough playing time.
Not really, one is old the other injury prone. So anything can happen next season. Also the gap in quality is not large, in fact I doubt there's a gap at all.
To the Mathie vs Bartra comparison - Mathieu is better when he has to defend players going outside, near the sideline. He is fast and has long legs so he's ideal choice to stop counterattacks through our left flank. But when he's forced to play centrally against teams that create and keep the ball, he makes a lot of mistakes.