Because after almost 600 episodes and virtually no in-world aging, a few jokes are bound to become predictions. This comes from Season 10's episode "Homer to Max," wherein the show jokes about what it'd be like to continue the show without Harry Shearer — which as of today is now a reality.
Here's the relevant bit of dialogue:
HOMER: This year's shows are classics. There's the laughter family. That's animated. Networks like animation because they don't have to pay the actors squat.
"FLANDERS" (with a new voice): Plus they can replace them and no one can tell the diddly-ifference!The only caveat we have here is that The Simpsons voice actors were paid... mmm... probably better than most out there, by as much as $440,000 per episode at times. Still, for most animated shows that haven't been around for 26 years, point taken.
And we didn't want to lead with this because EW did, but still... "The Simpsons did it." There, we said it.
Verge Video archive: The life of a voice actor as explained by Archer, Coach Mcguirk and Bob Belcher (2013)