This Is Water
This time around it doesn't look as if the DNC is going to rig the election. Warren and Sanders together double the numbers of Biden. If one of them drops out 90% of their voters will switch to the other. The democrat candidate will be a progressive.
I wouldn't bet on your Warren/Sanders math there. According to the polls I've seen, Biden is 2nd choice for a large chunk of both Warren and Sanders voters. If either of them drops out, their support will be split among the remaining candidates.