That was a cascading failure where the instability of part of the grid tripped protection systems intended to prevent damage to equipment throughout the rest. T

That was a cascading failure where the instability of part of the grid tripped protection systems intended to prevent damage to equipment throughout the rest. The individual regions making up that grid were perfectly capable of operating independently, possibly with reduced capacity, even after the blackout. What they couldn't do is remain operational whilst unexpectedly being cut off, because that caused transient unbalances between generation and load. I'm pretty sure there's no failure of a small number of transformers that would leave the US as a whole, or any large chunk of it, without power for a prolonged period. There are probably scenarios where it could trigger a cascading failure where protection devices shut down the rest of the grid in order to protect the other equipment from imbalances caused by the failures, but that alone is not going to lead to 12-18 months of blackouts and the death of 90% of the US population.