The PDI's biggest flaw was probably a mis-analysis of the personality types of pro bodybuilders.
In any other sport, the pros wouldn't stand for prize money increases smaller than the cost of living, or many of the other things they complain about. In EVERY other sport, they unionize to protect themselves. Pro BBers do not. So they'll continue to get exactly what they get.
IMO, if pro BBers had created a union when the PDI formed, they could have leveraged some longterm amicable agreements which could have helped them in the long run, using the PDI as the other party they could all go to. instead, they stayed fractured.
We know the company toolbox will attribute it to "PDI sucks!" but it's really the failut of the athletes. They had someone offering them the leverage to unite and gain all the rights they complain they don't have. They just weren't smart enough to realize it 
A union won't work, in my opinion, because the IFBB isn't the one shelling out the big $$$$$$. It's the supplement companies. They're the ones paying the six-figure salaries to the bodybuilders. Since you don't even need to be an IFBB pro to get signed by one, a union has little-to-no leverage. Besides, IFBB pros are quickly replaced. You will recall that 10 of the 13 guys that joined the WBF, years ago, were well-known and successful IFBB pros. When they jumped ship, other established pros (or new ones) stepped in and filled the void. The Matarazzos, Levrones and Yates' replaced the Strydoms, Christians, and M. Quinns of the pro scene.
Remember how MuscleTech started growing into the giant it is today. They initially featured one guy, working his way up to the pro ranks. It was following Greg Kovacs' journey to winning the Canadian Nationals that helped turn MuscleTech from a small company with a mere three supplements into the much loved/hated supplement empire that stands now. Mat Duvall, Idrise Ward-El, Mike Platz, Mike Dragna and Jeramy Freeman are just a few of the guys, who got signed as "amateurs" and won their pro cards and won them while on MuscleTech's payroll.
The same thing happened back in the day with popular amateurs from the early 90s. Dennis Newman and Paul DeMayo had deals before they turned pro (Parillo Performance and MET-Rx, respectively). In fact, some bodybuilders have more financial success chasing their pro cards, than they do once they get them and compete in the IFBB (i.e. Chris Cook). Before they won the USA and Nationals in 1994, Newman and DeMayo were making covers left and right, and appearing on bodybuilding shows like
American Muscle on a regular basis. I don't see how a union of IFBB pros stops that from happening.
Let's not forget about the guys who worked their way up the ranks, over the years. Ronnie Coleman turned pro somewhat inconspicuously (winning his class at the Universe), went for years without a contract, then signed with MET-Rx, followed by Weider, and finally by BSN. Of course, he won 8 Olympias and several other pro shows, along the way. But, he wasn't exactly a guy, pegged to dominate the pro ranks as were his contemporaries (Wheeler, Levrone, Ray, etc.).
Same goes for Darrem Charles. Looking at how often we see him in ads in recent years, it's easy to forget that he turned pro in 1992 but didn't get a contract of any sort until 2000 (with Weider) and didn't win his first show until 2002. He worked through the ranks and now, with his MuscleTech contract, he's doing quite well.
You need leverage for a union to be successful. Where such would originate, I don't know. You can be a professional bodybuilder WITHOUT having an IFBB pro card. Even if you have one, you don't have to be successful on the IFBB stage, to rake in the cash. Freeman has been/was with MuscleTech for nearly a decade; and he's hasn't come close to winnng a pro show. Duval has been/was with MuscleTech for about 8 years, with little success on the pro stage.
