Supervisory control and data acquisition (SCADA) hacking of power systems, pipelines, refineries and other large operations seems to be the greatest vulnerability that can be fixed IMO. Hardening transformers to be bullet proof would be a difficult undertaking. Power substations, like the example in this article could easily have protective walls and better security measures.
Zero chance you can harden all of these...even in a forty-year period. I grew up in a rural area...sub-station sat by a road, with a 1-star fence around it. Any idiot could have jumped the fence or blown up the station. Probably in same condition today.
Entirely correct about SCADA. I work in Telecom and wrote many papers on the subject. Utilities use the Internet to monitor their facilities. These can be easily hacked. There is a simple way to avoid this. Either install dedicated T1 lines to each site, or install dial back modems at each site. If T1 is used, hackers would have to physically tap in. If dial back modems are used, hackers would have to somehow spoof the dial back telephone numbers. Makes it difficult to hack either way.