First off,Chris had the "girlfriend problem". Tony has no way of knowing how much Adrianna told the Feds,or how much they had on Chris. After his confession scene in Tony's basement,he should have been disappeared with Adrianna. Any real Mob Boss would not have taken the chance of having two potentially damaging witnesses alive.
Second is the drunken tirade at the Bing. Again,any real Mob Boss would have had Chris killed for such conduct. However it did look like Tony was ready to pull the trigger until Blundetto stepped in.
And of course,we have Chris's drug addiction constantly being thrown in Tony's face
I wonder if Tony would have ever killed Chris if it were not for the wreck.
The crash basically gave Tony an easily plausible scenario for Chris's death. A tragic accident,nothing more.
I don't know if Tony would have wanted to have to explain away another disappearance like Ralph or Richie,so who knows if Chris would have ever been whacked,had he not crashed the SUV.