Their show was running alongside all the Discovery and History crap about ancient aliens, mermaids, and Bigfoot, so I'm not sure about those first two things.
In its first years, it actually ran alongside a lot of interesting and significantly more scholarly shows (than what we have now) on those two networks. The early 2000s actually had some solid programming on the history channel. Pretty quickly devolved into pawn stars and ancient aliens after that, though. So, yeah, half to most of its run was alongside utter garbage.
Edit for clarification: More scholarly than the current and last decade and a half of shows on history channel and discovery.
The same people working for David Zaslav who pushed discovery and history to be almost entirely pseudoscience and low effort variety/reality TV are currently running HBO's streaming service, Max.
Yeah, Adam did say they would never have the same opportunity today than they have in 2003, the landscape of edutainment show is just too different today.
They could probably do something similar with YouTube and a big patreon following. But they would have had trouble starting from scratch the way that Discovery’s production money allowed. Would have taken a lot longer to ramp up, but also a lot less lawyers would have been involved probably.