I do not have any legendary runes, so I cannot test this myself, but is it possible to select, say, one of the elite specialization runes without having completed said elite spec?
If so, then I believe that should be enough justification for those who have full legendary rune sets to get an unlimited and unrestricted legendary relic when it is available, because they already have access to runes they have not "earned".
If this is not the case, then at the very least, they should give these people something like the WvW provisioner gizmo that would give them whatever account-bound, zero merchant value, non-salvagable relic they desire which has been "earned". That way Anet does right by legendary rune set owners, but no one else but said legendary rune set owner can benefit (because of account-bound, zero merchant value, and non salvagable).