If you stop training, your abilities will fade. Maybe that black belt meant something when you got it, but if you haven't been training for a while, you may not be worth that piece of cloth around your waist. It's all the frills of the martial arts that throws off the effectiveness of the training. These are the easiest aspects of training and, unfortunately, the least combat oriented. People get caught up in belts, forms, fancy outfits, sifu this, and sensei that, because they are the easiest to practice.
Fighting is hard.
Training for it is always a struggle. Some instructors don't want hard fighting students because they chase away all the others. Let's say, for instance, that one in ten students has true fighting spirit and potential. One of ten does not pay the bills. This is how martial artists get a reputation for not being able to fight.