I think the a321 is good for maybe 70% of Monarchs short/medium haul but for some routes GIB comes to mind, that route was ideal for a 757 when the 320 was not enough capacity. The 757 can depart GIB with a full load, the 321 despite having less seats, cannot do this, there are other examples of the 321s limitations ithin the MON network, both charter and scheduled side. Sharm and some Greek and Turkish routes from Northern airports are best suited to the 757s excellent range and short field capability and also the ability to depart with no payload restrictions. I belive this is why Monarch keep hold of a few of the later build 757s in their fleet and I just do not know what the 757 replacement will be, the 321 cannot do the work of the 757 is some situations, True on short haul Spanish routes, the 321 is a cheaper option to the 757 bu there are many routes where the 757 is much more suited. Banjul is another route ideal for the 757, a 321 could not do the BJL from MAN with a full pax load the range of the 321 is not there.
The 757 is so well suited to many UK charter needs, that its telling that many of the 757s remain with MON, TCX, TOM, LS despite the 738 or 321 also being in their fleets.