how could the Rabanan decree such a penalty as to having a person keep his hand outstretched the entire shabbos? it would be impossible to fulfill! people have to eat, go to the bathroom, the weather might be inclement, etc. Why insitute a penalty that is impossible to fulfill? Additionally, if the person put his hand out intentionally, would he then obey this k'nas? And if he did it b'shogeg, why penalize him at all?
rziff, bklyn., n.y.
Besides what you ask, it is also not physically possible to stand twenty-four hours holding one's arms outstretched with (or without) something in it.
Remember that once a person cannot help letting the object fall (and he is an O'nes when he does), he will not be Chayav. In that case, it may well be that the Chachamim issued the K'nas anyway, forcing whoever transgresses to stand like that for one hour, two or three ... until he drops it, to remind people of the stringency of Shabbos.
be'Virchas Kol Tuv