I find the idea that Booker could cause more WWCs to move from Clinton to Trump to be extremely dubious -- by 2020 it'll be clear that Trump can't bring about his promises of bringing back manufacturing -- but even if this is the case the only states that would be put at risk are NH, ME, and perhaps WI -- not enough to break the Democratic victory on this map.
One of the key points is: for Democrats to win in 2020, no actual Trump voters from 2016 need to be persuaded over. It would help, but it's unnecessary. Better turnout or convincing third-party voters is sufficient.
The question is: did Hillary hit rock-bottom with WCWs in the Midwest? It clearly seems to be the case in Ohio and Pennsylvania, but whether that's true in MN, WI, and MI remains to be seen. Either way, your last point is quite true.
