That's the point, he focused on batting at #3, batting at #3 and keeping would be too much stress for one player to do it at peak, being the best batsman of the team, batting one down, keeping etc would be too much work for one man and who knows how long he will play? or for how long he'll keep his appetite to keep? you're welcome to assume he'd average 57 even if he had kept the gloves, but he didn't keep the gloves and when it comes to rating Sangakkara the fact is he kept exclusively pre prime and was entirely focused on batting after.
He kept for the first 35% of his career, and then moved entirely to batting, same as Walcott, had he kept the gloves he'd have been maybe the best WK bat ever, but he didn't and so he isn't, and that does not need his batting to be intrinsically effected by his keeping to be true.