Sg government probably learnt from the previous experience of Ang Peng Siong - one time Asian flying fish.
When he was at his prime (which was also the age to be enlisted), Ang Peng Siong had to serve national service without deferment. There, all the army training conflicted with the muscles set of a swimmer. He went downhill after that and never recovered.
Back then, Ang Peng Siong was Singapore best hope, but NS practically killed his sports career.