The problem with people who seem to think that Hillary will guarantee a victory for the Democrats in November is that they're viewing this election through 2008-tinted glasses rather than 2016 ones.
It is not the matter of Hillary guaranteeing anything: it is the matter of Sanders endangering it. A Trump presidency should be sufficiently frightening not to risk it.
If Sanders wins, it will also make it much more likely that Bloomberg enters: and, if he enters, that he will take a lot of Dem votes. he will not win, but he can elect Trump or Cruz.
If the primary is long, we will get a candidate AFTER the filling date to run as an independent in most states.