Flybe could quite easily replace the Guernsey based BAe 146 with a Dash Eight Q400, however two reasons come to mind why they don't want to :-
1) Alot of freight is transported in the hold of the 146, there is no way the Dash Eight could match it. As we all know freight makes airlines more money than passengers.
2) The loss of a jet service would send more passengers to Aurigny as you don't get charged all the extra's that Flybe charge.
In the Guernsey Press Flybe mentioned that they would also base the spare 146 in Guernsey, good luck to them finding a free stand!!