It's not unusual to count "thwarted aims" as a positive bad of death (as I've argued for myself in my paper Value Receptacles), which at least counts against replacing people with only slightly happier people (though still leaves open that it may be worthwhile to replace people with much happier people, if the extra happiness is sufficient to outweigh the harm of the first person's thwarted ends).
EDIT: Mestroyer was the first one to find a bug that breaks this idea. Only took a couple of hours, that's ethics for you. :)
In the last Stupid Questions Thread, solipsist asked
People raised valid points, such as ones about murder having generally bad effects on society, but most people probably have the intuition that murdering someone is bad even if the victim was a hermit whose death was never found out by anyone. It just occurred to me that the way to formalize this intuition would also solve more general problems with the way that the utility functions in utilitarianism (which I'll shorten to UFU from now on) behave.
Consider these commonly held intuitions: