How sure are you that what you are taught is a complete and unbiased analysis of political history, carried out by sufficiently smart and rational people that massive errors of interpretation are unlikely, and transmitted to you with high fidelity?

I don't think you have to be (certainly I am not,) not to put much credence in Reaction. From the premise that political history is conventionally taught in a biased and flawed manner, it does not follow that Reaction is unbiased or correct.

The tendency to see society as being in a constant state of decline, descending from some golden age, is positively ancient, and seems to be capable of arising even in cases where there is no real golden age to look back on, unless society really started going downhill with the invention of writing. There is no shortage of compelling biases to motivate individuals to adopt a Reactionary viewpoint, so for someone attempting to judge how likely the narrative is to be correct, they need to look, not for whether there are arguments for Reaction at all, but whether those arguments are significantly stronger than they would have predicted given a knowledge of how well people tend to support other ideologies outside the mainstream.