Years ago there was a thread on the old less-moderated /r/science about "Are there any of today's 'kooks' that you think might eventually be vindicated?" Hancock was one of the most popular answers, along with the two 'water weathering on the Sphinx' folks (forget their names).
Some of Hancock's ideas are a little wacky, but the core idea that civilization is older than we think probably has legs. Unfortunately the modern scientific establishment with its roots in the monastic system (academia as we know it is a direct evolution of the monastery and medieval guild systems) still persecutees heretics and tends to dismiss all works from those who espouse heresy. Having even a few "out there" ideas trashes a person's entire corpus and leads to a kind of excommunication.
The general trend I've noticed has been this: the more we learn about our history, the further back the dates go. I've seen this for both hominid evolution and civilization.
That and the standard narrative is just weird. We know from the fossil record that humans have had large craniums and by extension complex modern brains for at least hundreds of thousands of years. What the heck were we doing for all that time? Saying civilization is only ~10k years old is like saying "birds have had wings for millions of years but they only recently started flying." Huh? Really? This to me strains credulity more than the idea of lost civilizations and forgotten epochs. The latter seems like a straightforward possibility, especially if the end of the ice age involved catastrophic climate shifts that would have ruined agriculture, drowned coastal cities (we like to build on coasts), and dispersed people everywhere.
Even among recent ancient civilizations, only those that built massive stone megaliths are really remembered. We know little about native Americans that lived here only a few hundred years ago. There could have been massive and complex antediluvian civilizations and if they didn't happen to be in the habit of stacking up a lot of big rocks they'd have left almost nothing of themselves.
It's funny to me how much resistance this idea gets. I don't find it that hard to believe at all. I wonder if it conflicts with some residual "young-Earth creationism," even among some nominally secular folks. Even though most scientists no longer believe the Earth to be young, there still might be a kind of implicit bias against the idea that history might go back astronomically far into the past and that there might be forgotten epochs of history. None of that stuff is even hinted at in the Bible. I mean it talks about a big flood -- which if the ice age ended catastrophically would be somewhat accurate -- but has little if any detail about what came before.