Americans have lived without running water and electricity. They were called your great great grandparents.
Great great grandparents for me would be 1880’s. The population in 1880 was 50 million. You have to kill off 85% of the U.S. population, just to reach that level.
I think we are much tougher than the fear-mongering novels imply. But, given their proposed level of destruction of the grid and communications, a 90% drop is believable.
90%? That is preposterous.
And people aged very quickly and died a lot younger for a whole host of different reasons.