Aurigny are not buying a jet to simply maintain a jet service post-flybe. They are buying a jet to replace seats lost by flybe pulling out of gatwick. It was always about increasing Aurignys capacity.
Months of behind the scenes work has gone into researching what would be a viable aircraft for aurigny to operate with. The purchase was only happening because no other airline showed any interest at all in stepping in to operate this service.
Then a few weeks after the the aircraft is ordered, Easyjet knock on the door. What have they been doing for all these months? If Easyjet annouced they would continue the service when flybe left when the slot sale was publicised then you might find that aurigny would have not gone down this particular path and stuck to turboprops.