I came to the opposite conclusion about overlapping full backs. When we played five at the back under Trollope, it should've been the solution to the problem of where to play Whitts. An extra defender behind him so his defensive weaknesses wouldn't be so exposed while the wing backs could be picked out by his passes to give us an extra attacking threat. As we know, it didn't turn out like that, the wing backs barely got forward at all. I reasoned it wasn't because of a lack of belief that they wouldn't get the ball if they pushed on but a fear that they wouldn't be covered defensively.
I'm not blaming Whitts entirely for this but I'd certainly say he didn't help. Here's the late goal that Reading beat us with here last season (it comes at around 1.40).
https://www.youtube.com/watch?v=3wRj1Y1WyyI
Harris is left 2 on 1, I'm not sure if Whitts should have been helping him out (maybe that was Manga's fault) but I've no idea what he's doing. Noone is further up the pitch, suddenly realises the danger and races back, Ralls is already in the box with the other defenders but Whitts never appears. He could've tried to help Harris to block the cross, could've tracked the runners into the box, could've come back to make an extra defender but he does none of them, just stands outside the box and watches it all unfold.
Unfair to generalise but that example sums up what frustrated me with him in recent years. Not just the lack of urgency but the lack of athleticism too. He's 32, the same age as Ronaldo (and Bamba for us) yet we were meant to have sympathy because his legs had gone and it was the other players' responsibility to cover for him. As you say, his moving on is best for him and the club. Good luck to him at Blackburn.