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.
No they will hold the MPL until they have the flight experience to sit and pass an ATPL flight test. As per ICAO. The challange for most regulators is what should those flight experience requirements be?
The P in C must still hold an ATPL
My personal belief is the the MPL and direct entry into the airlines would be a good outcome for GA. Those who want to instruct or fly GA may be able to be paid better as all the hour builder pilots are removed from the lowest rungs of the aviation ladder.