The B757 age is a big problem as there is nothing to replace it at the moment.
The pb with the B757 is its fuel consumption which is 1/3 more than the A321. And in this days it is all about fuel.
However it does jobs that the A321 can't do or struggles to do, such SSH flights. The i/b flight may require a fuel stop usually in Italy and a change of crew due to duty hours on the A321. That means sending a full crew on 1 way last minute tickets....very expensive. This is using a A321 with additional tanks.
The B757 does it without pb.
BJL is out of reach for the a321, the B757 does it.
JSI-LGW non stop is not possible on a fully laden A321. It is on a B757.
The B737 offers more flexibility but it still has not got the ease of the B757 in fulfilling intermediate tasks such as mid haul or short take off....
That is the reason Monarch hangs on to its last 3 B757 but not for long unfortunately. They will be gone by nov 14.