If only 4 out of 100 Trump voters who thought in 2016 "I can vote for him, he will get more presidential" resign from voting for him in 2020, Trumps already done. The democratic base will turn out in record numbers and so on and so on. So the primaries are crucially important for the future outlook of the Democratic party, more so than the election themselves. And for progressives, it's probably the only shot they'll ever get to get someone progressive into the White House (because enough moderates will vote for them anyways). Which explains the visceral reactions against Biden, I think.