At the time of the Elko hire, we needed a full on shake up the D from scheme to recruiting to coaching. Kelly might not have thought that was Elston's skill set. A few years down the line, the D is looking good on scheme, fit and athletes, so a full shake up isn't needed. Maybe Kelly sees Elston as very suitable to take the D, already in good shape, and keep the engine running smoothly, maybe even a few tweaks to improve it. Or it could be that he liked Elston all along, but was blown away by Elko and then Lea. Can't blame him for liking them, they did a great job. Or maybe he doesn't think Elston is best suited as a co-ordinator and we won't hear him announced as DC.
Couldn't say whether he'd be a great co-ordinator as there's no way to judge, but he's been pretty good as a positional coach and with recruiting.