Trump will have a field day if they slip in anyone but Sanders at this point. Clinton can’t rig an election competently, anyone else will be easily accused of stealing the nomination from Sanders.
While Hillary has more actual votes than Sanders, the psychological effect of Hillary getting 500-700 superdelegates right from the beginning (between 20% and 33% of the needed votes) had a suppressing effect on Sanders’ support.
“Trump will have a field day...”
Look at the trend lines and extend them to November. Hillary will probably drop out in July or so because “Bill’s health is in trouble.” She has far too big an ego, and it’s fragile, to risk the defeat that’s shaping up to deep 6 her.
Republicans and Trump just need to make sure that he wins by an amount which will exceed the Margin of Fraud.