I take the Cardiff and Blackpool points but these are cases were Ryanair has abandoned those airports completely and are, I agree, probably more suited to a smaller aircraft serving them anyway. If Clickair tried to return on the Barcelona route or Flybe on Leeds, Ryanair would immediately increase their frequencies again so it's a totally different scenario. Actually Leeds is probably the most unfortunate UK airport affected by the Irish tax as it falls just outside the €2 bracket and is subject to the €10 tax.
Ryanair will have selected to cut marginal routes for them where they don't expect to see a reaction of increased competition. Yes, they may have had to be cut from the summer schedule anyway but surely you can accept how a €10 increase in charges will accelerate this given the amount of taxes and charges they already absorb in their free/€5/€10 offers already.