But if policy matters so little, then why did these voters (who presumably voted for the Republicans in nearly all elections pre-2016) change their votes in 2016? I guess Trump's behaviour played a bigger role than his policies but if these voters weren't tribalistically Republican enough to vote Trump in 2016 I doubt they will become tribalistically Democratic to the point they're not even going to vote for a Pence/Cotton/Haley anymore even though they would have voted for such a candidate in 2016. The only way I see these people remaining Democratic is if they like some progressives fear "screw us on policy" (and at that point I'd probably also become a Democrat ).
It's quite possible a lot of the ongoing change is related to white Millennial grads continuing to displace older white college grads. Last I recall from an Atlantic piece, the education divide is strong among Millennials as well, so it makes perfect sense to think that as they grow up, it would shift white college graduates along with them. As for white college grads that were previously Republican-leaning, it's also not a stretch to think that their connection to the GOP was already on tenuous ground pre-Trump, kind of like Democrats and some WWCs. Obama being president and rather unpopular most of the time made it easy for them to maintain that connection. Now that that is no longer the case, and the spotlight is on a deeply unpopular and personally repulsive Republican president, it's allowed the separation between various white college grads and the party to solidify. It's similar to how in Virginia, many Republican delegates held Obama/Clinton districts - some even double digit Obama districts, and yet they kept winning because an unpopular Democratic president demoralized Democrats and made it hard for Virginia Democratic candidates win over other voters. Now that the script has flipped, a massive correction occurred and districts Republicans were destined to lose finally fell.
I actually posted an article with some polling data on Trump supporter's extensive malleability with policy preferences, depending on what they were told Trump supported. The gist was that if Trump supported more liberal policies, these voters would also support more liberal policies, and vice versa. This isn't a new concept, either. It's been shown that many voters will take cues from the political leaders they support. You may think of it along the lines as,
"I support the Republican Party, and I'm not a policy expert. If they say this will help, I believe them." Of course, there are some exceptions. Immigration seems to be one for some voters, although I'm not sure to what extent. For instance it doesn't seem most Trump supporters care about his wall, and I think most even supported a pathway to citizenship.