I believe the MAX deal was tied into some direly needed funds - which came via Boeing. So Monarch are wedded to the marriage with the MAX. They say to marry once for money...
Of course operationally the Airbus fleet isn't old, is pretty fuel and maintenance efficient. What we don't know is what the lease terms are like - I suspect there is a whiff of the type of thing that dragged bmi down once it was separated from its original owner. The group had various associated leasing companies from which aircraft were leased to the operating airline at rates which were not necessarily representative of the market. This seems a sensible (if not terribly ethical) way to move profits around and works as long as the group remains intact. Once separated off the money becomes "real" and the obvious problems start.