BE are openly critical of the runway and stopped running a jet service to Gatwick. Most locals chose Aurigny with their more modern planes.
I was almost tempted to believe you until that bit. The E195 (brand new jet) could operate into Guernsey, and given some of the loads I've seen on the Dash, I'm sure Flybe would love to use it (as happens in Jersey). The problem is with the airport's infrastructure, and until that's upgraded, the E195 can't get in there.
And as for 'more modern planes', Flybe's Dashes are still coming off the production line. How much more modern do you want?