End of the year may be a good time for him to go - if he fails to make the O2 then he can refer to the back injury to save a bit of face and slip away without any further embarrassments. He can bang on all he wants about playing Rio in 2016, but his current form shows that he'd be a shell of his former self by 2015 at least.
It's a shame in a way, as he's been the main player in this recent 'Golden Era' for tennis, but I won't miss the media fawning over him and his fans turning a blind eye to his many arrogant proclamations.
I go along with that too. It will seem a bit strange without him because he's been around for so long, but I'd rather see him go with a degree of dignity rather than just slide further and further down the rankings.