Is Hillary Clinton at fault for Obama losing in 2008? No, because Obama won by a landslide despite a much more negative primary against Hillary Clinton. And so Bernie Sanders was not at fault for Clinton's loss, she is much more at fault herself.
The difference is that Obama attacked Hillary from the Left. Left-wingers are more likely to be independents. Hillary's 08 voters were old school democratic loyalists. By the way, It was shaping up to be a close race in 2008 until the economic collapse. Obama didn't lead Mccain once in FL or VA or NC, etc. until Lehman Brothers collapsed.
This is also the same dynamic with Trump, Trump picked up voters in the GOP primary who were proportionally more likely to be independents, and were least likely to be GOP loyalists, this is why the assumption that any other GOP primary opponent would win off of the same Trump voters is also absurd.