I should have phrased what I meant better. Individually, she may not be each candidate's second choice, but in those matchups you can totally see how she's each "lanes" (if they exist?) 2nd choice.
If you are a Klob/Biden/Pete voter as a group, and you lose all three of those options, you're left with Warren or Bernie. Warren's an easier swallow for those folks. Likewise, if you're a Bernie supporter, and he is out of the race for whatever reason, and everyone other than one of Klob/Pete/Biden is out of the race, then Warren is your natural second choice.
What we know, empirically, about 2nd choice is all we know from Iowa. Basically, very few people actually went to Bernie in round two versus everyone else. Warren supporters tended to go Pete and Klob, which is the reciprocal of what I'm saying, to a degree. I genuinely think if we removed one of Biden, Pete or Amy this race would look drastically different. Pete would have won NH and iowa (by a larger margin). This divided field is really the main reason we're talking about Bernie as a favorite to win.
Like I said above, I should have said it was "2nd choice by lane." (insofar as lanes exist, I guess.) I should have explained better. Hopefully the next post I made made it a bit clearer.
Maybe I should have said Warren is the candidate most people would be second okay with.