This. Martin was not the cause of his collapse, but he also didn't help Djokovic get out of it.
I think one of the errors in that coaching relationship is that Martin perhaps pressed Novak *too much* regarding coming to net. Djokovic will always be best from the baseline, with just the occasional foray to close at net when he is well set up for it. Martin wanted to see too much net play out of a guy who just isn't very naturally gifted from that part of the court.
Also I'm pretty sure Djokovic hasn't lost in a GS R16 in many many years... AO '07 maybe?
I think it's rather about perfecting one aspect of your game, before moving on. Both Federer and Nadal have taken this approach. Novak lost more than a year trying to improve his net play and in the process lost confidence with his basic game.
This year should be about tentatively improving these aspects, without losing direction and a comfort zone to fall back to that works on most occasions.