You don't become "unstoppable" on the same day your opponent sweeps the Midwest.
You're talking about optics. Obviously they would be good for Bernie and bad for Hillary if that happened. I'm talking about math.
If Bernie wins IL/MO/OH by the same margin he won Michigan while Hillary wins FL/NC by 20 points, she nets ~50-60 delegates. Add to her current 220 delegate lead, and that's all she wrote. He needs to perform respectably in at least one of FL/NC.
He could win every other region of the country and still lose the nomination because that's how proportional allocation works. Narrow wins throughout the Midwest don't do him much good when he's already trailing by so much. Just ask Hillary Clinton...in 2008.