As soon as this becomes apparent, it ceases to become an 'accident' investigation and becomes a criminal one
Apparent to who? The point you are missing is that it can't be "apparent" until the investigation is complete. If the result is prejudged as "apparent" after only a few days then the whole investigation is potentially flawed as the evidence can be selected to fit the "crime" and evidence that doesn't fit is ignored or given little credibility.
To say that as soon as any crime is suspected means that any accident investigation then goes out of the window is to completely miss the point of protecting pilots from consequences if they "fess up". If you ever get the chance to visit the AAIB down in Farnborough please take it up - perhaps then you will understand the point I'm making.