Some dubbers actually become irreplaceable in the eyes of the public, but most of the time it happens because of how charismatic they are and their presence outside of the dub, like conventions or fan interaction, that is why Vic is still getting invites and still atracting huge numbers.
In Mexico this is very noticeable, again, DB Kai was a complete failure, mostly because of censorship and not using the original cast.
The Netflix re... something of Saint Seiya had the main character replaced because the actor died, the trailer had more dislikes that the English version and half the comments were of people complaining about the replacement for Seiya not being good enough.
The guy who plays James in Pokemon has been the same guy all the time in Mexico if he was replaced without a good reason you can bet your ass people would make a big deal out of that.
Dub actors can become irreplaceable, but it's up to the actor to get that status, Schemmel doesn't have the personality to capitalize on his Goku Voice and become iconic, he is way too subdued and irritable to do it.