I agree with some of your comments, yes the A321 can not completely replace what the B757 can do.
However its operating costs are way higher than the A321, hence MON and TCX using them.
By the way the MON B757 are not 83 vintage but 87/88, so are not that much older than the A300-600s they operate.
Now as for TCX, you forgot the fact that they are going to replace the B757s with a fleet of Airbus A320 Family aircraft.
