"What happens to Babylon 5 if...say, you're walking down the street
and a wandering asteroid crash lands on you, three years into the story
arc?"

What happens is this:

First they scrape up my remains. Then they put me in the morgue.
After a cursory examination, the death is ruled an act of god, which is
the ultimate insult to an atheist. Then they stick a big needle in one
arm, and in my heel, and embalm what's left of me. Then there's a
service. All of my friends come. What they do with the rest of the rows
after one is filled is anyone's guess. Rent them out, I guess. Next they
plant me. I lay there for a while, gradually coming to the realization
that when they stuck me in this crummy suit, they yanked the pants into
my butt and now I have to go through all eternity with a snuggie. In time
I get really annoyed by this, my essence rises up out of the coffin, AND
HAUNTS YOU FOR THE REST OF YOUR NATURAL LIFE!

Or, put another way, if you want to see the end of the story, it's
in your vested interest to keep my ass ALIVE for the next 4.5 years.

Ever read Sheherazade? Think about it.

jm(rosebud)s