Because maybe the Jays sign him? Just using the Dodgers as an example, in 4-5 years who knows what team it will be. My point is someone would be willing to take him on if we had to offload him, but who's to say he's going to be ineffective in 5-7 years? There have been pitchers that have been valuable in later years -- Randy Johnson, Nolan Ryan, etc ...not comparing Price to those guys but it's not just a given that he's going to be an albatross.
Plus, we don't have many trade chips left and I don't know of better free agent pitchers that going to be available, so when you have a chance to sign an ace you do it.