they retired Bart’s teacher Edna Krabappel as a character after Marcia Wallace, who voiced her, died. They then just had to avoid scenes where they had to show who Bart’s teacher was for 9 years until they finally created a new character. Aside from knowing that she was retired as a character, I learned that by looking it up just now. I can’t even imagine the show working without having scenes of Bart in class, which I guess shows how far it’s strayed from what I think The Simpsons is supposed to be.

At this point I don’t see how re-voicing the characters makes things that much worse, but it looks like they probably woudln’t do it. So I think the show continues until someone who voices characters they think they can’t make the show without retires or dies.
I think that aside from the people who voice the four Simpsons who talk, they can’t make the show without Harry Shearer. Losing Hank Azaria would mean losing Moe. I can’t imagine the show without Moe’s Tavern, but they went 9 years without showing Bart interact with his teacher, which seems equally unimaginable to me, so who knows.

You can see who voices who here: https://en.wikipedia.org/wiki/List_of_The_Simpsons_cast_members