"It wouldn't be a Victorian novel if it didn't have a controversial will in it, would it?" John J. Miller, "The Great Books."
This is true, and not to be disdained. It is a great plot device, as it relies so strongly on its place in a society bound by laws, and traditions, and sense of honor that we know every codicil will be enforced without provoking rebellion. Yet it also sets in high relief that many people will choose something other than money, standing on principle or declaring their independence. They will refuse to marry, or will marry someone who does not meet the will's requirements. Or the device will set into high relief the character's lack of principles, of scheming to find a way to meet the conditions while subverting them.
The murder mystery only sells in places where people do not think they are in much danger of being murdered. It's not entertaining if there are actually thugs ruling the county.