Its fairly realistic though isn't it?
Calta Vigo got relegated in their first season in the champions league, Betis have struggled this year and even Villareal are only 9th.
Middlesboroughs league form has been a lot worse then there European form.
Surely if your players weren't used to playing in Europe they would get worked up for the big European nights and then suffer in their domestic form?
Plus if you have a small squad then trying to play on 3 fronts or more is going to cause a lot of injuries.
I don't think its so much a question of cups sucking as where you want your priorities to lie. Picking and choosing where to concentrate your resources is what being a mid-rank top-flight club is all about as you wont be able to compete with the big boys all the time.