I think he had another twelve months left in him but I guess that's how the old saying goes, "Always leave them wanting more". Given his injury history, Kompany knows his body better than anybody else - he's likely spent the last two months asking himself whether he can demand more playing time and produce the same quality for another year, but has failed to come up with an answer.
We'll know more when the second part of his statement is released, but I imagine he watched as Zabaleta and Yaya struggled to maintain their highest levels during their twilight years at City and realised he didn't want the same to happen to him. I worry about our leadership and calmness out on the field without him there, but if we can keep Otamendi or move Fernandinho into the back four then we should be able to do okay.