Only one person on earth can be Mr. Burns, and a lot of people want to see Mr. Burns. So if you want Mr. Burns on your show, you had better pay that person a lot of money.
I'd be willing to bet the opposite is true -- there are a LOT of people out there who can do a completely convincing Mr. Burns. I doubt his main characters will even be retired -- they'll just have somebody else voice them.
When Mel Blanc died, did Warner retire Bugs Bunny, Daffy Duck, Porky Pig, Tweety Bird, Sylvester the Cat, Yosemite Sam, Foghorn Leghorn, Marvin the Martian, Pepe Le Pew, Speedy Gonzales, Wile E. Coyote or the Tasmanian Devil? Nope. Take a look at Wiki and you'll find that even the legendary Blanc's characters were able to be mimicked by a raft of other voiceover artists, and some of those artists were able to cover not just a few but many of his characters.
The fact is that like any other job, voiceovers are something that can be done very well by a whole lot of people.