Our generation has become more sedentary in its lifestyle. This invariably means a lack of exercise for the body muscles. Human body is such that whatever one does not use, becomes defunct. Regular exercise is therefore very essential for everybody. There are three types of physical exercises - for Strength, Endurance and Flexibility.
Yoga provides only flexibility type of exercise.
Hence, yoga alone is not enough. When yoga was evolved a few thousand years back, the lifestyle was not sedentary and perhaps in those days, yoga could be enough. Not any more. Every individual is unique. This fact should be remembered.