I think the election of Donald Trump has thoroughly obsolesced that theory. I agree though, that Bernie will need to focus his rhetoric in ways that cater to older voters. He shouldn’t waste time during a primetime debate educating us on the positives of the Castro government. But, he also shouldn’t call for an uninspiring return to “normalcy” that is said to have existed prior to 2016.
In my opinion, he should make the argument that in order for a return to so-called normalcy, change must be made now in order to get there. From there he should thread a throughline where he rhetorically employs the likeness of the FDR, Eisenhower, LBJ, and JFK. Remind voters of a nostalgic time when the middle-class exploded and America achieved great things “not because they were easy but because they were hard”.
Indeed, Bernie does astoundingly well with voters under 40 of all ethnicities, genders, races, etc. But those young voters are, unfortunately, for many reasons only a small slice of the electorate. Catering to the older, much larger voting bloc, is the path forward. And, I happen to believe it is very much possible.