We are in the era of mega orders ( e.g. Air India and Turkish recently) but one possible problem is relying on a single type . We all know the training /maintenance /spares huge benefits but you are also at the mercy of a single AD , and the Max had its share .
Most Legacy airlines evenly split their fleets between A and B.
i also wonder what will happen if , for the sake of the discussion , another Max would go down due to a trim issue.
RYR is also betting on pax looking at the ticket price rather than the type of aircraft when making a booking and they are right so far . But i know quite a lot of people that would definitively stop Flying a Max if there was a 3rd accident . Just ask around .