Agreed. A more sane patent system would require a working demonstration, and further require that the demonstration must be performed each time the patent holder claims infringement. Any idea that can not be readily demonstrated in a practical sense should not be patentable.
What about things that are beyond the current technology, like Babbage's analytical engine? In general I agree with you, but should a numerical simulation or logical proof of feasibility be able to substitute for a demo?
No demo, no patent.