Precisely. But it gets even worse. There's nothing in it for the UCI, ASO, etc. either. If truly 'everyone' is juiced and they all come out, the sport will be completely and utterly destroyed because sponsor money will evaporate, TV coverage will mostly go away and the general interest will disappear. Sponsors are fine with the smoke-and-mirror tactic which makes the stars appear to be clean (as long as the general public buys it). If this appearance goes away (see Rasmussen) things can get ugly really fast. If the whole sport is perceived to be too dirty (and cycling is somewhere near or over this threshold) or proven dirty (as no doubt would happen in the case of an amnesty), it will be the end of it for a long time.
So, maybe I didn't address your question 'Should there be an amnesty', but I can answer a different question 'Will there be an amnesty' with a resounding no!