I don't disagree that the crowd sizes aren't necessarily indicative of what's going to happen, but I think the thing that kills the Obama Romney comparison is that Romney was not the incumbent and never had the enthusiasm Trump has. Also Romney had zero chance at chipping away at Obama's minority votes. Obama's base always loved him and that's not true of Biden, he's been more convenient than anything and basically had to be show horned into the nomination (which I think matters a lot more than people realize).