It's surprising how many people don't know how planes work, but you'd think pilots would understand.
The problem (For most people, at least) wasn't that they didn't know how planes work, but that they didn't know quite what exactly the question was asking in the first place.
I always assumed the conveyor belt was just a way of saying "the plane is being kept stationary, can it still take off?" and if you look at the vast majority of the arguments against the plane taking off, they all talk about air flow and such.
If the plane is moving forward, there will be air flow, if not, there wont (and it wont take off). The myth was testing if the conveyor would stop the plane moving forward, that's clear now, but a lot of us (myself included) thought the myth was testing if the plane could take off from a stationary position.
The end result? Most people weren't really WRONG, it was more a case of whoever came up with the question didn't explain it well enough.