And as for 'more modern planes', Flybe's Dashes are still coming off the production line. How much more modern do you want?
Trouble is most people think if it's got big spinny things on the front it can't possibly be new!
Well I use Flybe perhaps twice a year so I can't claim to be a regular passenger. However I like the convenience of Southampton to Limoges to see the parents, BE have never let me down, are consistently cheaper and I have had excellent service at every point. (And I preferred using them over my own carrier's staff travel as it was often cheaper and easier!)
Hand baggage size wise I quite happily get my roll aboard in the overhead bins of the Q400. It takes a good weeks worth of clothes so quite what everyone else feels the need to have with them on a regional flight is beyond me.
As to the OP all I can say is whilst it is a shame you felt the experience unecessary, perhaps a more careful perusal of the terms of carriage of BE might have reduced your frustration. The 10kg in hand baggage is a gimme in terms of standards weights. The 20kg you have in the back end is what you are limited to. Simples.
If passengers are more pre occupied with undertaking the miracle of flight for the same price as a Big Mac meal in many cases or a return ticket to London on the train from Bristol at the top end then please don't expect the niceties of a premium carrier at the same time. As MOL would say - very very cheap and very little service - tough - people keep flocking back though.