There is just a point where you're old enough to just go ahead and enjoy life rather than stress your body. Athletes can choose to go out when their bodies are still pretty decent or you can retire because the body has become unable to keep going, if you're already rich, risking life long joint problems by going longer than you need to honestly just isn't worth it.
He could be a quad gold medal winner if he really wanted, but the older you get, the extra years of training, the worse recovery from injuries and the more stress on joints, end up richer, but risking joint problems for life.
Many footballers and other athletes end up spending half their life in serious pain from all the stress they put into their body, 99% of them would exchange the last few years of their career for better health.
Honestly I can't quite recall the gap in the 100m, but the 200m.... Bolt still has a lot of time and pace he can lose before the rest catch up let alone beat him. He may be too old by the next olympics to win, he might not, but there is nothing that suggests the next year or two will suddenly be hugely difficult for him.
But he's already at that point where he has money, has all the fame, everyone believes he's the fastest without question. He's a personality, he can take commentary/pundit jobs for easy money without all the training, without the risk to future health.