I think it's fair to say that failures at certain tasks suggest those tasks are harder than people imagine. They are certainly a reason to be wary and to look harder, and to learn the lessons from a failure. But they don't prove something impossible.

On the other hand, it can also be foolish to assume that the reason everybody before you failed at something was because they were all stupider than you. Sometimes that's true but not always. The baggage system was not a PRT, but it contained some technologies people want to use in PRT. Certainly a working PRT system could be easily reworked to deliver bags to destinations as readily as it delivers people. Probably too expensive a way to do it, though. Deliverying people should be much more expensive than bags in almost every way, and cheaper only because people enter and leave the cars under their own power.