I'm also giving the nod to GSM's laying out of the issues. Most of us probably believe that this lay-off is more psychological than physical. The guy was clearly in need of a serious re-set. And I get why Britbox poses the question: it's been a long and mysterious walkabout. I agree with those that say it's hard not to see him finding his way at least part-way back to his A-game, though he was showing signs of losing a step, which was affecting his timing, and a bit of loss of power on his shots. Could be age, or it could be lack of desire...probably a bit of both.
Darth is a task master, and thinks the motivation should be obvious. Sure...he's set his sights on Rafa and Roger, and who are the two guys that took best advantage of his slump? Also, they are two examples of how to adjust your game post-30. He might find inspiration and motivation in that, or he might not. Plus, he's got Agassi for that very purpose. I'd be shocked if he never wins another Major, but I'm waiting to see how he comes back to guess just how great his late career will be.