It actually says on Wikipedia that Blink-182 pulled out after people had already gotten there (really, it took them that long to figure out it was a disaster?)
"The first flights from Miami International Airport to Exuma International Airport, operated by Swift Air and Xtra Airways, landed at 6:20 a.m. That afternoon, Blink-182 announced that it was withdrawing from the festival, stating in a Twitter post that: "We're not confident that we would have what we need to give you the quality of performances we always give our fans.""
I think McFarland did on some level think he was going to pull it off. I think part of it was probably an "Emperor's New Clothes" situation, and part was an extreme power of positive thinking gone wrong situation, with just straight up delusional levels of overconfidence and narcissism. I think a normal person would not be able to understand his mindset. But basically it's like he formed sort of a cult where he brainwashed not only other people, but his own damn self.
I think Delvey definitely has to be worse though. Both of them scammed investors and business contacts, but on the other hand you could say the investors and business contacts failed to do due diligence before getting involved with them. McFarland also scammed festival attendees but at least they just had one or two slightly uncomfortable days and then were left with a good story (and the people who spent their life savings on tickets were just idiots). Of course he did endanger lives by not giving people access to water or medical attention, so there's also that.
However, Delvey scammed unsuspecting people who she had supposedly befriended, even depleting them of their life savings in some cases. That's like a real backstabbing, sociopathic thing to do and could ruin someone's trust in people forever. Like how can someone enjoy a luxury vacation knowing that in order to pay for it, they're about to trash the finances of an unsuspecting person who trusted and cared about them? I just cannot understand that.