As I understand it though, it's not like paying for bad service at a restaurant. I know there are no guarantees with good outcomes in therapy, of course - BUT there should be a guarantee of professional and ethical behaviour from the therapist.

If they do not act in a professional manner (ie termination over the phone, no referral to another therapist - all stonkingly bad form imo) then I'd argue they can't expect to be paid as a professional. I pay for a professional service - if the therapist has bigged themselves up and bitten off more than they can chew, instead of behaving in a responsible and ethical fashion by referring the client to someone with the depth and breadth of experience to be able to help, then no, I would not pay, and would feel no moral implications.