The language definition is pre-existent to the PEG parser you can write for it. PEG has a lot of advantages as a tool, and its a nice experience to write by hand. You do not author a PEG script, you author a language definition first.
You're saying that if the lang definition is lost, then is hard to recover it from the PEG script or the parser code. Yes, but it is not a normal situation.
You're safe if the grammar is LL(1) -- what you sees is what you get. After that it gets mysterious fast.