> Here the key question here is: what sort of things should you expect to have already seen, if they were really there? On the one hand, we clearly have seen enough to safely conclude that there aren’t large dinosaurs roaming the streets in our major city centers. On the other hand, we often hear reports of people uncovering old things that others had confidently predicted would never be found. Which makes many suspect widespread overconfidence in claims about what we know can’t be there, because if so we would have seen them already.

This wording is very reminiscent of a lot of writing from conspiracy theorists. It starts with a very plausible claim that experts are sometimes overconfident in a mainstream belief about topic X, but then immediately leaps to the conclusion that we can therefore make up any claim whatsoever about topic X and it ought to be treated just as seriously as mainstream beliefs held by experts.

You see this all the time in government/political conspiracy theories. Yes, we have in the past uncovered real substantial government conspiracies and attempted cover-ups. But that doesn't mean that any claim whatsoever about a government conspiracy is true or should be treated "with the same priors" as any other claim. Likewise, the fact that some experts have been wrong about what artifacts they expected to find in a certain archaeological dig does not mean that all claims about human history should be treated with the same level of seriousness. The fact that sometimes evidence is hidden (either deliberately, or due to lack of resources, or due to misguided research, etc.) does not support any arbitrary claim made without evidence of its own.