You are explaining why at the time it seemed like the right thing to do. You are missing the point which is that chassidus was not another ST. The Gra did not know that at the time. There are two possible explanations for this error, either he misinterpreted the information that he got or he did not get good information. Choose one of the two.
Of course chasidus isn't another ST. Its the total opposite.
However when trying to understand the picture, the events of that time period gives you an idea.
It has nothing to do with the greatness of the Gra. Chasidim were making changes to things that scared many to the extent that they believed that it was only a matter of time before it became a ST. I don't understand why you consider it derogatory to the Gra. He was situated in Vilna while the closest chosid was in White Russia. Additionally there were many ppl that had a hatred towards chasidus According to he was told he believed that it was apikorses.