I don’t think she will step down for any reason - but if they tried to put warren in there I think the Bernie bots would explode! They would, rightfully so, feel he should be then next in line.
The only decision that matters is what Obama and the DNC cook up.
Bernie can rant and rave all he wants....he is irrelevant.
They had to threaten him to get him out of the primary race against Hillary.