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Iranian Hackers Probe US Infrastructure Targets
Taco Cowboy points out reports in The Register and The Jerusalem Post (along with a paywalled article at the WSJ) that say "[Iranian hackers are] responsible for a wave of computer attacks on U.S. corporations, with targets including oil, gas and electricity companies. Unlike the cyber incursions from China, the goal of the Iranian attacks is sabotage rather than espionage. The cyber attacks are seen as attempts to gain control of critical processing systems. The attacks on oil, gas and power firms have so far concentrated on accruing information on how their systems work – a likely first step in a co-ordinated campaign that would eventually result in attacks aimed at disrupting or destroying such infrastructure."
The steps you mention are good ones, but an air gap is still a very good step in that defense in depth approach.
Maybe in some situations. In others it can make the situation worse. If you disconnect everything, and have to send out a truck to make an adjustment at a substation, then you have a problem when there is a big storm and not enough trucks. For most sensibly designed systems, disconnecting from the network will likely cause more problems than it will prevent.
However, they don't necessarily guard against interruption of service.
I once worked on a control system for a hydroelectric dam. The software could adjust the gates to control the flow of water to adapt to electrical demand, but only within certain limits, which were set depending on expected demand. To go outside those limits, a worker had to manually extract and reinsert a steel rod. It is also common in coal/gas/nuke plants to require manual intervention to shutdown a generator, or even reduce the power into the "brown-out" zone. Since that is something that will almost never need to happen, requiring manual intervention is reasonable. Designing a system to prevent a denial of service is harder than just preventing catastrophic failure, but it is still possible.