I'm not so sure about the lack of motivation.
He's always come across as a very hard worker.
Surely the thought of playing the top flight players of today and continually getting thrashed is almost as unpleasant as losing in the early rounds to weaker players. The sting of losing coupled with the drop in rankings must be extremely painful to a former champ.
Would love to see him stick around, but I'm not sure how long he can take the pain. At least Euroka will be happy if Andy (not Murray) retires!