Isn't one way of looking at it that they are having to give up their cosy country life in order to get the best for their child, and that in itself is a valid storyline?
Mike said as much, quite poignantly I thought- "I've had my life, it's Bethany's turn now".
I really felt for him- all he wanted was companionship in his widowerhood and for a while it was all going so well, and now he has to change everything for a child he never planned to have; he must wonder if he made a mistake taking up with Vicky. [I hope that parents of children with SEN don't feel offended by this, he is a fictional character and really did think his fatherhood days were over].
It's quite funny, what with this being a radio show, that characters who have moved away can no longer feature unless they visit the village- it's not as if they'd have to build a new set to have a scene where, say, Hayley goes to visit them in Brum. There is precedent for out-of-Ambridge scenes- remember Pat and Tony meeting Rich at the Christmas market in (I think) Leeds? Also, I'd imagine that Mike and Vicky would come back to Ambridge often, for country air, catching up with friends and seeing Abbie and Phoebe. Would be easy to keep their story lines live.