For once I actually understand Ryanair.
1: they run a Business, if the FAA and EASA and all the others aprove the aircraft, it is deemed safe.
2: there is no way to assign a particular aircraft to a flight a few months in advance. What happens with AOGs, diversions etc etc.
Im sure other airlines will do the same, of realize that last minute changes forces them to use a MAX instead of a NG