Actually, Yanyan was not really worried about the future situation of Bingzhou, especially since he knew very well that King Zheng of Qin would be the one to successfully achieve the grand cause of unification.
Given the personality of Ying Zheng, unless Bingzhou proactively provoked the State of Qin, Qin would only tacitly acknowledge the existence of Bingzhou.
Have you not seen? Even the tiny State of Wey, though later destroyed by the State of Zhao, was still restored by Ying Zheng and continued to exist until the demise of the State of Qin.
Chapter 312