Replacing the fleet with 737 Max will reduce the company's operating costs by 24%. It's not hard to see why they want to go down that route.
The saving calculations are based on the existing fleet, not a fleet of new Airbus products.
While the 24% saving is highly likely, flying a like for like route, in comparison to a 10 year old A320 for example, in maintenance and fuel costs.
But again I think this is highly imaginative accounting on Monarch's part, in the press releases where this figure is mentioned...
Monarch are only going to be receiving 6-8 aircraft a year, so will take nearly 4 years for the transition to take place from Airbus to Boeing. In this time they will be running a dual fleet, they have to pay for crew and ground staff to be converted, and equipment at bases to be changed too.
I highly doubt this 24% saving will be achievable until the Airbus fleet has been fully disposed of.
While these new aircraft will extremely fuel efficient, this comes at the cost of very high prices to purchase the aircraft and/or lease. This is where low cost carriers like Allegiant and Jet2 have decided to operate older/cheaper aircraft and the cheaper running costs out weigh the fuel savings of newer aircraft. Hence why Jet2 have ordered the previous generation 737-800's and not the max version.
While the A320/1 family Monarch operate, varied with different ages and slightly different spes, this is common for many airlines, where additional aircraft of the same type is acquired over the years.... As long as they have the same engine type and basic specifications they are more or less flying a uniform fleet.
An order for new aircraft, for a struggling airline is the last thing they need and should be trying to make do with what they have for the next few years, until they are out of the woods. But I suspect this order is too far advance and the noose around the airlines neck, hence the struggle to find a new investor or buyer.