That rare beast, Just One Cycle Man, is hard to spot in the wild. Your gains, natty or enhanced, are dictated by your diet and your training. That said, if you go the enhanced route early then your baseline body is sculpted in an environment that is by definition unnatural for your biology. Maintaining muscle mass that was achieved in a supraphysiological state is significantly harder than if you did so naturally.
Think of it like this: I give you a nice street car that you take to the track every weekend. It’s got a good, peppy naturally aspirated engine and is suitable for racing. You have it for a few weekends and have some fun. Then I put a turbo on it and tell you to go nuts. Over the course of the next year you drive it faster and better each weekend. By the end of the year you’ve got a solid lap time and you’ve learned a lot about driving it. Then I take the turbo off and you go back to a good-but-not-great engine. Those skills you learned will always be with you. You’ll be a better driver than you were at the start. But you’ll never beat your best lap time without that turbo.