I would imagine Flybe don't really care "what IS required to make a good Captain in an operation like this", but they do have a fair idea of what is required to make a good captain in their own operation.
I understand your point and your frustration, but there has to be a cut off somewhere. I would imagine Flybe have analysed their previous recruitment successes and rejections and set their requirements (for DEC!!) accordingly. No one is suggesting that anyone is a less capable aviator simply because of the weight of their aircraft, but they have decided on a profile they are looking for - and for whatever reason they want experience on an aircraft more than 10 tonnes, to be a DEC.
Moving further along the line you'll find that Jet2 do not deem Dash 8 PIC time as good enough for their DEC recruitment profile. In fact, in the recent past this experience would only get you a 70% FO contract. Is that right? Well, again I wouldn't want to be thrown into the left hand seat of a 737 from a Dash 8 LHS no matter about ground/sim/line training. But maybe that's just me.