The nightmare, essentially, is Hillary. The argument is that Mrs. Clinton could win the presidency by a much larger margin than has been seen in decades. That could result in a down-ticket blow out for Republicans in key states. The way this piece lays it out: Clinton cruises to the nomination, and currently looks unbeatable against a number of possible Republican challengers.
She wins in a landslide bringing up the rest of the Democratic ticket with her. It's very possible it could work that way. The question is this: is it probable? It's really too early to say. I'm now leaning that Clinton will run but I still don't think it's a lock. Her days a State are more problematic than the left and their friends in the media want to admit.
Further, her age and health are indeed legitimate questions. Voters will ask them even if the left/MSM scream sexism. As this piece also points out, the competence of the Republican campaign will be key. And the "relatability" factor will be critical. President Obama owned it over Mitt Romney in 2012. Hillary isn't Obama in that regard. That said, she currently is much stronger than he is in approval ratings.
It could happen this way. But Hillary has to run first. We'll see.