Odds Favor a Hillary Clinton Candidacy
Securing the nomination means you are halfway there, and the wide-open Republican nominating contest along with the divisions that exist among Republicans -- establishment and the Tea Party, social conservatives and libertarians, foreign policy hawks and isolationists -- argue for the fact that Republican unity for the general election is not a given, as was the case in 2008 and 2012. This too justifies the long odds against winning the presidency for any individual Republican (which may sap the determination of good candidates from even competing for the nomination), or for any Democrat other than Clinton.
Clinton is not a lock by any means in the general election, and her long political history provides plenty of material for whomever her opponent will be, assuming she runs. Her Republican opponent will also be very well-funded. One would hope that the technology divide that was on display between the two parties in 2012 has been eliminated or reduced, which would assist Republicans in targeting and get-out-the-vote operations.
But in making the decision to run, the near certainty that she will be nominated if she goes for it is enough reason to think this will trump all other considerations.