Listen, I feel a bit unsure of myself - suggesting that negative evolution be somehow equal with positive evolution, but unqualified.
The point about negative evolution, is that it doesn't guarantee survival on the basis of aggressive predation. If positive evolution comes up against negative, positive will win, in every circumstance but where positive is overdone (if you will).
The point I believe it makes, is that you cannot always rely on positive evolution, when in fact negative evolution can take the same contenders and give an advantage above and beyond being "overdone". If you are negative and you are "overdone" on the other hand, you are left without enough adaptation to meet the challenge. Both positive and negative being "overdone" it is the stronger that wins, once again.
What you can't do, is rule negative evolution "out": it has a specific function, that is congenial with positive evolution - to a greater degree, not a lesser one.
Maybe that will go someway to explaining negative evolution, as fits with your (evolutionary) "schema".