Eh? I can understand finishing a book series off after an author dies but while they're still living then it's up to them. What you're suggesting is a bit like if Axl had asked a G'n'R covers band to come in and finish off Chinese Democracy for them.
The whole point is that it's more than the story and even someone imitating a style isn't the person creating it.
As it stands he's said he doesn't want it finished off if (when? ) he dies before it's complete and maybe that's because he's read the completed Wheel of Time stuff and feels it's compromised by the new author, even if most readers might not feel that way.