If Obama decided not to run, the only way another Dem could win would be to renounce Obama’s policies and run to the right of the Republican candidate, which ain’t going to happen.
Actually, this could play out a lot like the Dem campaign did in '68. A lot of people don't remember that Johnson actually started running for reelection that year. Eugene McCarthy lost, but made a much stronger showing than expected in the NH primary by running to the left of LBJ. This showed some fractures in LBJ's armor, which resulted in Bobby Kennedy being drafted and making a run up the middle.
I could see a Kucinich, Nader etc., actually trying a run to the left of Obama and getting some of the far left base, then Hillary being brought in to "save the party," with Obama being discreetly asked to exit stage left.