To be clear, Hillary Clinton wouldn’t have needed to act as a chicken little saying that the sky is falling. She had a program that was mostly based on reforms that make sense and addressed real concerns.
The great strategic error was, simply, she didn’t make it real for the Rustbelt. It’s really a failure to take basic Strunk-and-White style seriously: take a Clinton speech, and it’s full of abstract nouns and not concrete ones, the verbs are all bureaucratic (funding this, legislating that) but lack activity (contrast with Trump’s vivid language about draining the swamp).
The endgame Ohio strategy reflected that. Cleveland is somewhat revived, and yes people love the Cavs now. But doing a grip-and-grin with Lebron James doesn’t address basic concerns of black urban Cleveland/Akronites, much less the concerns of the much more desperate areas of Ohio which became Pillbilly Trump country. It wasn’t enough that the governor was a die-hard Never Trumper — plenty of GOP’ers who wouldn’t vote for Kasich in a non-presidential year turned out this time around.