I would not call STUXNET proof that this can be done "easily" -- that was joint work by the NSA and Israeli intelligence, who are among the best hackers in the world, and it used 4 separate 0days simultaneously.

While you certainly make a good point about the difficulties that come with securing the laptops and other equipment, the military has the same problem. Most of the users of classified military systems are not themselves experts in computer security. The necessary training is not a graduate level course in security; it is training people to always match cable colors to port colors, to only use laptops provisioned by the organization, etc. The military disables, sometimes by physical means, USB ports on laptops that are used on classified networks to close one of the key security holes Stuxnet took advantage of.

Sure, it is challenging, but this is not impossible to solve. Utilities would have to be forced to do it and they would need time to overhaul their operations, but they certainly can do it, and there is an argument that the NSA's IA division should help (as the electrical grid is a critical for national security). It can be done, we just have to be willing to do it (and pay the cost of doing it).