Bhishma was a man out of his time, having lived far beyond his generation, clinging to the old values and in many ways preventing true progress. Think of it as an old and powerful Director of a Board supporting a useless chairman just because of a promise he made to the former chairman. The company was rotting, greedy members had come on board and the chairman’s greedy son was truly running things. Cousins who had an equal stake in the company were tricked out of their stake and exiled from the company. The board was running the company indiscriminately with no concern for the shareholders, the customers or the general public.
In the end, they had to trick Bhishma to get him out of his position and literally pin him to the ground so that they could make way for progress.