Many people really struggle deciding when is the right time to retire their dog from agility (or any other demanding dog sport). It can be a tough decision especially in a sport that you and your dog really love. Some people are forced into the decision if their dog becomes injured. Other people avoid the decision altogether and continue to compete with their dog long after their dog really should be, often covering up their dog's discomfort with meds so that he can make it through a training session or a weekend of competing. Everyone has their own reasons for doing what they do and I really do think most people make the decision that is best for their dog, not just for them.
I decided to retire Bosley from agility last year. It was an easy decision. He had accomplished more in agility then I ever imagined, being that he is a large, not-so-driven dog. Although Bosley still liked training, his weakening pasterns had me concerned. Being a dog that weighs nearly 100 pounds, the strain of agility was taking its toll. He never had any major injury, which is most likely due to his conditioning and that he is a well built, balanced dog. But, his pasterns had given him some trouble in the past and I decided to retire him rather than risk them getting worse or causing a more serious problem somewhere else on his body.
Bosley has not done any agility (except for a few jumps and tunnels in our yard) since early last summer. Since then, I have noticed that his pasterns are looking better and it no longer looks like he is walking with his pasterns on the ground. They are not the same as they were when he was young but they have improved a lot since he has not been jumping and straining them so much.
I am sure Bosley doesn't care that he is not doing agility anymore. He still has other things he is training in and as long as he gets his turn doing something, he is happy and content. At the end of the day, that is all that really matters.