I read today that more millennials voted for Sanders in the primary that Clinton and Trump combined. Whatever happened in the Democratic primary, it seems pretty obvious to me that Sanders had broader appeal to the general electorate, and he remains a much more popular politician than either of the two GE candidates.
Linked for everybody else-
https://www.washingtonpost.com/news...ers-than-trump-and-clinton-combined-by-a-lot/
The problem with your analysis is that millenials only cast ~10% of the vote in the Democratic primary... 3M out of 30M... Not to mention that Trump is Prez only by dint of an anomalous result in the Electoral College.