As I wrote in the other thread, I don't actually believe that the "original arc" as explained in that thread is what would have happened if O'Hare had stayed on the show. So many of the changes have nothing to do with Sinclair. Most of them probably would have happened anyway. It's also unclear when some of these changes to the arc were decided on. Some of the changes were probably already in progress during Season 1, when O'Hare was still there.
The biggest change from the original arc to the one that we actually got is that the original arc ends the series on a cliffhanger, with the conflict with the Shadows unresolved. I speculated that one thing that might have caused JMS to change his mind is his realization of what can be done with CG effects and what can be done on a TV budget.
The way JMS pitched the series initially, he made it sound like the whole story would be told from the POV of the station, whereas in the show as filmed, we have many of our characters going off and doing important things throughout the galaxy in Seasons 3, 4, and 5. We also have a lot of (by early 1990s standards) complex space battles and action scenes. So I wonder if JMS just didn't think that the Shadow War as he envisioned it could be realized on a TV budget in the 1990s. So he figured "OK, we'll make this five year story called Babylon 5. If it's a huge hit, then we'll do the spinoff series, and by then the technology will catch up, and my imagination can run wild." But then once the show got into production, and he realized how he could stretch those dollars and show things with CG that he hadn't anticipated, that freed him up to include much more within the 5 year arc of B5.
That's pure speculation on my part, but it seems plausible to me.
