Took the wrong Duke guy (Okafor and not Tatum), and those two decisions taken together probably cost the Sixers a chance at being the franchise to take over once the Warriors are done. That franchise will almost definitely be the Celtics, who reaped the benefits of choosing the right Duke guy (Irving) in a franchise-altering trade, and again in the draft (swindling the Sixers for Tatum). Duke has its fingerprints all over the highest levels of the NBA for the next several years.
If Porzingis and Tatum are on the Sixers instead of Okafor and Fultz, the next 10 years would've been more or less decided already, barring injuries. 4-5 more Warriors titles, followed by 5-6 Sixers titles. PG Tyus Jones (FA) or Tre Jones (draft), SG Jayson Tatum, SF Ben Simmons, PF Kristaps Porzingis, C Joel Embiid. Instead, the Sixers basically have two long-term franchise pieces at this point and will not have any more easy ones coming to them, now that they're entering mid-first-round purgatory.
There haven't actually been that many elite level busts coming out of Duke in the past decade (going back to Shelden Williams at #5 in 2006). Okafor really stands out. The Duke top 5 picks since Shelden have been Irving, Okafor, Parker, Ingram, Tatum. We could even say "top 9" instead of top 5 if we want to be cherry-picking dicks about it. Of those five top 5 picks, Irving is a star on both ends of the court now, Parker was proving to be something very interesting before injuries, Ingram is coming along well, and Tatum is maybe the best 19 year old ever. Okafor was a bad pick at the time and a bad pick in retrospect, which will make it difficult for any team to justify taking a not-amazing defensive center in the top 10 again.