A philosophy differs from an ideology. Long-termism is a philosophy, meaning an abstract framework that is used an intellectual aid. An ideology, on the otherhand, is an all-encompassing world view, that inserts itself at the top of one's value hierarchy, and allows one to do as you describe: define any wrong as a necessary evil to fulfil the greater good.
The reason long-termism is merely a philosophy, and will remain so, is that it doesn't provide a narrative that lends itself to ideological zealotry. Ideological narratives are simplistic models of the world that can be easily digested, and provide a ready litmus test to categorize any person into one of three groups: victims, villains, or saviors.
In my opinion, the tendency of the optimal action for short-term interests to coincide with the optimal action for long-term interests, and how much easier it is to determine the former over the latter, makes it better to focus primarily on the near future, with a less-well-defined long-term objective being used only to orient one's general direction.