What would That Great Rabbi have Poskened?

I’ve been following several discussions online that express an idea which makes no sense to me. Several of the posters, not all in agreement, explain what they are sure some great halachic expert would rule were he alive today – either on a new subject, or on a subject where they have already poskened, but new data is available.
Now it seems to me that part of what has made these illustrious Rabbis so great in halachic reasoning, is that they have often surprised everyone with the brilliance and clarity of their arguments that reach an unexpected conclusion. I might personally claim that I know how perfectly ordinary Rabbi X might rule on a question, but my inability to predict what a Gadol haDor might rule, were I ever so much more knowledgeable, is simply part of what made them great.

I can recall a similar situation in medicine. A surgeon counseled a rather extreme operation for a certain condition. He encouraged us to consult with the surgeon well-known as one of the two greatest experts in the field, but he said, “and I know what he will say! He’ll tell you that this is the operation you need.”
The great expert counseled no operation at all, rather alternative treatment that made the operation quite successfully unnecessary. If the first surgeon had really known what the second one was going to say, he would have had to be an equal or greater expert in the field, not just someone familiar with his work.