So your saying Joe Biden would be a stronger candidate than O'Malley, Warren, Gillibrand and Klobuchar? A guy who would be 73 on ED and makes frequent gaffes every time he speaks... Gillibrand and Klobuchar are the Dems strongest non Hillary candidates any Republican would struggle against either two.
I'm not even convinced Hillary would be stronger than Klobuchar in a general election; more strengths but also more weaknesses. People are also exaggerating Warren's weakness in a general election, as many did with Obama ahead of 2008 when he won 365 EVs. What about her exactly is going to scare voters more than the GOP does? You think people in swing states love income inequality or something?
I'd personally give the GOP a 75% chance at winning without Hillary running. Given these Senate primaries and the tendency of an out-party to moderate(see Silver article on this) after 8 years, I think an establishment candidate like Walker or Jeb will get nominated.
The out party has moderated when less moderate nominees lost preceding elections. But out Republicans nominated Reagan and out Democrats nominated Obama. The GOP is coming off 2 moderates losing in a row. Also, they'd be emboldened without Hillary running and today's GOP has a stronger base than parties usually do. Don't buy the spin on the senate primaries. These GOP nominees are still very conservative albeit better packaged, the equivalent of Pence, Walker, Ryan. The GOP race is too unpredictable to say one way or another which way they'll go (or would go without Hillary running).