A large part of the reason that happened has to be laid at the feet of Khosh and the Vorons though. Sheridan only got into the position he did, (and by extension Delen), because they set him up as the leader of the alliance vs the shadows. The ISA was basically a peacetime continuation of that. G'Kar obviously got catapult to where he was as a result of the Narn Centauri conflict, and Lando was the obvious candidate after Mr. morden was done manipulating things. None of them where actively seeking to take the whole thing over, even Lando was just looking for more power rather than absolute power. They where all to varying degree's victims of the fallout between the shadow/vorlon manipulations and what happened after they left. Overall it worked out for them, but none of them would have gotten where they where except Delen if it hadn't been for those two.
I think the most interesting question though are the one's we'll never have answers to. Over the course of the show a number of characters left. Others came in to fill the holes and performed the same general roles, but at the same time the details naturally differed. Seeing how those stories would have differed would have been very interesting. There's also that age old Lando/G'Kar question too. According to JMS when he was writing the initial premise he flipped a coin to determine which would be the evil one and which would be the good one. But who won and who lost?
Crusade also raises so many questions, even though i think they dangled the cure right in front of our noses in the episodes we did see, we never got to see several aspects of the story, a number of side plots where just starting and we never got to see where those where going.