Conventional wisdom at the time was also overly conservative about how computer technology would unfold in many ways. They didn't see desktops coming at all. HAL's components, in the book and film, were also really large. When something like HAL does roll around, it'll probably be a lot smaller.
My dad used to do programming on really large machines. He used to bring home used punch cards for me and my brother to play with when we were kids. I imagine that Arthur C. Clarke, Stanley Kubrick, and the other people who made 2001
were thinking of something along those lines in terms of size. I agree that if and when we do get something like HAL, its components will be much smaller that what those folks had in mind. Then again, I could be wrong - it seems like making a prediction about the future is an almost certain guarantee that things won't turn out that way. Personally I find this exciting.
When I was a kid pretty much everybody I knew expected the USSR to be around indefinitely. Then the early 90s came, and suddenly it was gone.