I don't know if Hillary's on that level. Historical trends suggest that a party does worse in the third term seeking the White House than in the second term. Even Papa Bush's 40 state win with a national margin of 7.8% paled in comparison to Reagan's 18.2% in 1984. Plus, it isn't clear that Democrats can be as effective without Obama on the ballot. It would be in keeping in historical trends for Hillary to lose the General Election by about five points.
Your reasoning makes sense to me, but... who is she going to lose to? (Presuming she does get the Democratic nomination in the first place.) Christie seems to be out for the foreseeable future. Most of the nationally prominent Republicans are popular with their fans but seem to be unelectable nationally even if they
don't self-destruct while campaigning, as seems all too likely. Who is left? An aging Jeb Bush (whose wife doesn't appear to want him to run)? Or some competent and charismatic dark horse who's going to pull a Jimmy Carter? The last seems the most likely, but I'll be damned if I can see who that's going to be. XD