No, you won't lose it all, and for most people, if you keep training, you'll stay above your previous genetic limit. I'll say for damn sure, my genetic potential was not 180lbs at 5'5" with 12%bf. My dad is 5'10" and has never been over 175, he usually sits around 160. Through usage of gear like tren and IGF which increase hyperplasia, I think I increased my genetic max. While steroid-induced muscle SIZE may go away, steroid-induced muscle FIBERS themselves won't just dissappear or be reabsorbed. If you go from 100k to 130k fibers in a muscle from IGF, and then stop all gear, those fibers may shrink but they won't dissappear. If you keep training and eating right, you'll keep plenty of it.