I think Hillary would beat Warren, obviously. But as I've said with Romney, people won't start taking her seriously until she acts like she wants it. If Warren said tomorrow that she was considering a run, her numbers would spike. People just don't want to put their weight behind someone who doesn't want to run.
Exactly, if a credible challenger were to put their hat into the ring Hillary's Soviet-esque poll numbers would deflate rather quickly back to normal poll numbers. The media in constant search for election drama would rush to give the first serious challenger to Clinton ridiculous amounts of airtime and attention.
Fortunately for Clinton, a credible challenger doesn't seem to be in the cards.