Rebellion? Shaking your fist under the nose of Leviathan. Without the First coming to their rescue, they would have been wiped from existence. Heroic, yes. Brave, yes. But hardly the triumph over adversity I could have connected with. I would never count on someone stronger to solve my problems for me. Instead, I would try to make myself stronger to survive.
Realizing that, it should be no mystery as to my disaffection with the ending of what was else-wise an excellent series.
When the Shadows said "they will not follow you if you are dead" and tried to destroy the White Star and the cruiser did the only thing it could do to defend them and moved between them.... That was when it was won. Vorlons and Shadows could make people die. They could not make people die for them. Even if they wiped out every single one of the younger races they could not win because the younger races would no longer submit. That is triumph over adversity beyond winning by force of arms.
The First One simply helped them realize it was over. When no matter what you do you will never get what you want there is nothing left to be won. And while the Vorlons and Shadows were petty enough to have wiped everyone out from pure spite they were not going to do it with the First One watching.