What would happen to Smallville if...

I don't know if this has been asked before, but what would happen to Smallville if one of the lead characters, Tom Welling, Michael Rosenbaum or Kristin Kreuk died?

I know it isn't something you want to think about, but what would happen to Smallville?
Are they going to find somebody who takes over the part or would the show just end without us ever knowing the ending?