Artest will likely not be a huge black hole if he comes to LA. If he does that he simply won't play. It's not like Houston where they had nobody else to handle the ball in the clutch.
In LA Ron will have a very well defined role as a 3rd/4th option at best. If he cannot accept that then he will not play, period. Given Phil's history with coaching unruly players I would be surprised if Artest acts out and plays himself off the team.
The bottom line is that Artest is a bigger, stronger, and better defender than Ariza is. If you didn't notice, Ariza got abused by both Ron Artest and Melo in the playoffs. He can't handle bulky SF's like Pierce or Lebron the way Artest can. He's also a better shooter than Ariza, a better passer, and more capable of creating for himself.
There is obviously going to be some risk, but the Lakers are not a stupid franchise. They don't have 15 rings for nothing.