The substation is the switching and routing element, and taking that out is just as effective as taking out the generation, and likely considerably easier.
The transformers they damaged, if the windings went, could be replaced by spares. The main tranformers at a power plant which connect the generator to the grid are long lead time items. Most power plants do not have spare main step up transformers setting around. This is an example of a main transformer being transported by a schnabel car. As you can see, these are much different than those in substations.