Forgive me if I've missed something, but I thought that bmi as a group ended its Star Alliance membership just before it was sold by Lufthansa to IAG last month. Thus regional's relationship with Brussels, SAS and Swiss is that of codesharing rather than full airline alliance membership.
As such, Brussels, SAS and Swiss will feel rather less obligation in the future to continue being buddies with regional if they don't think it's in their interest, particularly as Brussels+Swiss's parent in the form of Lufthansa group no longer has an equity stake in regional.