I think that the 70 years thing is also to dis-incentivize killing the creator to cash in on his work a mere 10 years later. Though 70 years is overkill even in this regard, I doubt anyone would wait say, 20-30 years, for such a plan.
That may have been an argument in setting the date, but Disney was the real driver of it. The reason it's 70 years after the death of the creator is because Mickey Mouse was in danger of becoming public domain again, so they added years to the law.
I think you'd be hard pressed to find any instances of people committing murder for IP reasons.
