I may be completely off track here, but my understanding of the MPL is that holders of that particular 'licence' will never occupy the Left seat of anything. They are FO's for life. Correct me if I'm wrong, but on that basis, there is still going to be a need for Pilots to occupy the Left seat to keep those MPL's honest. Those pilots have to come from somewhere and as sure as all hell, it ain't going to be from an MPL programme.
Far from this being the death-knell for GA, it is just another diversion in the road. There will still be a need for qualified and competent GA drivers well into the future.