I just feel like many airlines doesn't recognize the MPL for their applicants, most just say CPL ME/IR in their ads. But maybe that is because MPL is probably extremely uncommon in their applicants. Ryanair for example does not hold the MPL as a valid certificate in their cadet program..
Regarding the CPL conversion, i'd keep my rating valid and when you're "unlocking" the MPL for single pilot ops with the CPL the IR/ME comes with it. And everything would be kept valid with the yearly 737 PC, this is why i am considering it. Not sure anyway, one of the good parts is that I would be able to fly single pilot IFR to keep current with those parts and IFR time feels like it would be more attractive to a possible employer. Not sure though as I said, never been to a real interview except the one for school where the airline essentially recruited us.