This borders on epistemological nihilism. Of course few assertions about truth should be beyond challenge. Of course there are grades of confidence and properly conducted discourse (including venues/methods of challenge) can improve the quality of conclusions. That doesn't mean there's no such thing as poor-grade or even garbage assertions.

And functional institutions not only have a responsibility to try and establish some kind of reasonable consensus about the truth, they basically can't operate without doing so.

When courts make rulings, they can't do it without establishing facts of a case. That's not "presuming" so much as it is simply functioning. There's a process for arguing details out... and an end at which details are accepted as established. Potentially subject to review, but not arbitrarily.