Look at bands like Lynyrd Skynyrd, I think there's no original member left.
There's a lot of bands like Megadeth, but also Opeth (Mikael Akerfeldt), Pain Of Salvation (Daniel Gildenlöw), Porcupine Tree (Steven Wilson) and others where one man calls the shots and one can't really imagine that they could exist without them. But the Spock's Beard example shows that it can happen and even work to some degree.
Doesn't mean that it always makes sense or that the outcome is always good, but never say never when it comes to replacing band members.