Having grandchildren “coast through life” is based on copyright lasting 70 years past the death of the author. But seriously having the rights disappear in 10 years is hardly an incentive for murder.

Honestly, I find it difficult to understand why a fixed 40 year term isn’t long enough to benefit from copyright. Trademark is already indefinite, JK Rowling is hardly going to be meaningfully harmed if someone publishes a work based on the first Harry Potter book in 2037. Less wealthy authors generally need to keep working anyway. Publish a hit at 22 and perhaps it’s time to start saving for retirement just like everyone else.

Another point for the copyright term being a fixed 5~10 years. The current system already incentivizes such agressive tactics to anyone with sufficient patience. If a teenager's favorite book has just been written by a young adult, they only have one course of action if they want to live to see it in the public domain for a few years.

> there would be unwholesome motivations.

Which are life imprisonment for murder. Not some magical "my children must be fed millions without ever working until 70 years after my death".

Are there any notable instances of murder for copyright reasons?

The current law is still extends the copyright of a work until a time after the author's death. So if one wished to hasten the expiration of those rights, the motivation still exists; although perhaps diminished by a 70 year wait.

Well, after accomplishing the author's untimely demise, the murderer (or facilitator) would have to wait 70 years to profit (unless 70-years future contracts on copyright expirations are a thing, I wouldn't know)

Seems a lot of risk and effort for a small chance of profit.