Trump is a nativist, unpredictable, egomaniac wild card, more like Le Pen than anyone in American history. Sanders maybe a bit like Pepe Mujica of Uruguay.
If they both somehow get nominated by the Republican and Democrats, all the major powers in American politics would have no one to support -- no finance puppet, no one to pretend to care about the religious right, no reliable corporatist.
That would leave a huge hole in the center that some Democrat or moderate Republican would like to fill as a third party candidate.
If the centrist were a Democrat, the left of the Democratic party would be furious. We went along with Clinton, although we didn't really like his Third Way policies, we endured a compromised Obama who had to make due with what he could get, while expanding the imperial presidency and continuing permanent war, and then the minute we get our guy on the ticket, the other side splits. It would ruin the Democratic party. And we love Sanders.
In the centrist were a Republican, it wouldn't matter that much. They would be no more divided than they are already and could keep going more or less on the same trajectory.
Would the Democrats tolerate and Sanders win? If he won, would the NSA let him defund them? Would the CIA put up with him?
Bernie himself seems to think he can just say the truth, stick to his guns, win on the basis of the truth, and then institute sane policies, as if we lived in a constitutional democracy governed by the rule of law. If he thinks that's all true -- our democracy isn't completely a sham, there is no deep state, etc. -- maybe he's right. We'll see, hopefully.