Probably depends on Mirka and the kids, as he said it couple of times. As long as the pro tour is not a burden on his family life he no doubt will be up there and enjoying it. He has gas left in the tank, not to dominate of course like he used to but always to be dangerous in all tournaments he enters, bar clay.
The pressure is truly off I reckon and his gazelle like movement will allow him to age a bit like Rosewall and stay relatively in form and injury free most of the time, along with his smart scheduling.
The rivals are also not up to par with Nadal, Djokovic, etc still the main players to beat in big tournaments. The young guns are just not delivering in terms of pushing those older guys off the throne, not to mention the lost generation (Nishikori, Raonic, Dimitrov, etc) who I thing didn't live to the expectations.
Once again, Mirka and the kids are the biggest factor in his late career - not only now but after 30. People don't talk about that usually, but the support he's getting from his family to concentrate solely on his pro career is immense..