“…if Democrats want to win in 2020, making nice with Sanders and his wing is critical.”
It is indeed imperative that the Sanders wing be brought into the Democratic fold. But it appears that they will settle for nothing less than his nomination. Democrats have embraced many of his ideals, so in that sense he has already won a huge victory. Yet my impression is that the advocates of Mr. Sanders would rather see Trump in the White House than any Democrat other than Mr. Sanders.
I have challenged Sanders supporters to pledge to enthusiastically support whomever gets the Democratic nomination. So far, not one of them has agreed to that pledge.