> where you have control over your estate via the will, maximally until some time after the death of a specific named person who was alive at the time of your death. (which I find a strange stipulation)

This is a compromise of allowing some of your wishes to be fulfilled after your death while preventing long term 'dead hand' control over property and resources. You can set up a foundation dedicated to "thing" but can't control what that foundation does until the end of time.

We probably don't want to live in a world that is controlled by the wishes of someone gone for 500 years who had a view of the world completely at odds with the present. It centralizes power and ties the hands of those living in the present.