Why not focus on those Sanders voters?
-Because they were at a far lower risk of flipping to Trump.
It was a rhetorical question; my point being that basically all the polls showed that people who flipped to Trump weren't Sanders supporters. (Clinton did hemorrhage some Sanders supporters, but more went to non-voting or third-party support than to Trump). The voters Trump picked up were mostly those who frequently hadn't voted since 2008. The idea of there being a large (or even small) number of people whose preferences went 1-Sanders, 2-Trump, 3-Clinton has been disproved numerous times. (That plenty, maybe most of Trump supporters went 1-Trump, 2-Sanders, 3-Clinton is of course known, but that wouldn't really help the Democratic nominee).