I think it as telling as anything. People putting their money behind her is a BIG deal.
Once the name the VP, given them 2-3 weeks of constant rallies, TV appearances and interviews and then check back. If they knock it out of the park in those 3 weeks and then give strong debate performances, I think they'd have a very real chance of winning.
I also think it shouldn't be discounted how many people truly hate Trump and are embarrassed by him. Giving them even a halfway serviceable alternative would be enough for a decent sized subset of that group.