Defensively, I trust Paul Pierce in big moments. Maybe he's noticeably slower than he was in, say, 2004, but he's about the same guy, athletically speaking, that he's he's been since the 2009 season as far as I can tell. He's a ridiculously smart and skilled basketball player on both ends of the court. Personally, unless he suffers another serious injury, I'm not too worried about him.
As far as the turnovers when driving through traffic go, those have always been something you accept as a part of Paul Pierce's game. And, fans have always complained about it. The way I see it, this is something that we need to accept as a by-product of his aggressiveness.
If he stops going to the hard into the lane, then those turnovers will decrease. I don't want him to ever stop going hard into the lane.