The further problem with the MPL is that he is a EXTREME "low timer"! While a simulator is fine for learning procedures, it does not give a pilot the same "air sense" as a real airplane. The MPL graduate does not have enough time in real airplanes to qualify for a CPL or an IR!
While virtually anyone can sit in a modern airliner right seat, read checklists, and monitor instruments during routine operations, it is the times when equipment fails or real emergencies exist that the MPL will be at the greatest DISadvantage! His experience and air sense will be so limited as to significantly decrease the safety margin built into the current system.
The problem will be exacerbated in the future when the now-new MPLs are at the point in their career when a traditional FO is ready to upgrade to Captain. The MPL will not be qualified, because he will not have the CPL/IR or ATPL, and will not have the PIC time (since I am not familiar with the "PICUS" concept, I can't say whether the Europeans will count that as real PIC time) to qualify for either of them!
IMO, the MPL concept may make short-term financial sense for airline management that wants to grow quickly in new markets, but does not make sense as a long-term solution.