I think Monarch have a problem because what aircraft can replace those old 757s? newer second hand 757s maybe, late build models. The 321 even in favourable conditions will struggle with a full pax payload and a full compliment of pax baggage. SSH from say Man, most are holidaymakers on 1 week/2 week hols so most if not all will take hold luggage, which will cause probs for the 321 when operating these red sea charters.
There is no suitable 757 replacement in current manufacture. 321 is less capacity, people think its almost a 757 seating but it has fewer seats and less range, The 757 has been good for Monarch as they could use them on short haul, medium haul and even longer haul charters, with no payload problems. I feel moving to an all 321 fleet is what mon want but they recognise, clearly that the for some routes the 321 is not the right choice, hence why they seem so reluctant to part with those few remaining 757s