Well for Icelandair they already have orders for the 737-9MAX (correct me if I'm wrong) to replace the aging 757 fleet, which makes sense.
Now with Monarch I'm guessing Boeing gave them a better offer or waiting time than Airbus would. As for the maintenance side of things Monarch's maintenance do an awful lot of work on Boeings, here's the last few aircraft to enter the hangar at BHX:
G-POWD B767 Titan, ZZ337 A332 RAF, EI-LNA B787 Norweigan, G-POWH B752 Titan, G-POWD B763 Titan, EC-ISY B752 Privilege Club, G-ZBAL A321 Monarch, G-ZBAO A321 Monarch, ES-AEB E170 Estonian, G-ZBAJ A321 Monarch, G-ZBAR A320 Monarch, G-ZBAF A321 Monarch, G-ZBAI A321 Monarch, G-ZBAM A321 Monarch, G-ZBAK A321 Monarch, ES-AEB E170 Estonian.
So yeah, it does seem that Monarch would be more than able to cope switching to Boeing.