Why would they bother giving him any more screen time, atleast past him making announcements for his Shadow overlords? We know that whatever he does he ends up as a broken old man who only gets kind of redeemed at the end, after almost 20 years of siding with evil. There is no room for any character growth, we know who he turns out to be.
heh. so much for how you thought it would shake out. there was a ton of character growth for Londo between "War Without End" and "The Fall of Centauri Prime". He has saved his planet twice. After Sheridan, he is the second most responsible individual for the rise of the Interstallar Alliance (bringing the League of Non-Aligned Worlds together to back Sheridan against Earth). In the end, Londo is not siding with evil Shadow overlords. He has no choice but to sacrifice his freedom to save his planet and bide his time till he can act. 20 years of siding with evil? not at all. quite the opposite.