You could just as easily handwave away Trump’s winning deeply red states, like Texas. Clinton won by two, and the polls mostly had in her in that range. And Trump was never quite the underdog he likes to portray himself as. Fivethirtyeight had his chances at 30%, which was similar to the betting odds at the time. That’s not exactly “Dewey defeats Truman” territory.
So when the polls say millennials are abandoning the GOP, and the GOP keep losing races it has no business losing (e.g., Penn special district, Alabama senate seat, a bunch of state seats), it’s pretty clear the GOP is in trouble.