The way the party is going, Bernie will probably step in. Trade one fossil for another and run him from behind the scenes as originally planned.
I think it will be Hillary before Sanders.
Physically and mentally Bernie ain’t Joe. Marxist politics aside he is still sharp and his exaggerated Brooklyn accent will be a formidable debate foe. Trump will need to beat him totally on the merits. And Bernie will have moderators and instant analyzers totally on his side.