With Genesis it was explained that it only worked for Spock because his body landed there at the right moment and the device was made with banned matter that caused the whole planet to blow up. So you would have to blow up a planet every time you wanted someone back and you would run the risk of them aging to death. Then you have the issue of memories. The new person looks the same but they don't have the memories. Spock only got his memories back because he put them in McCoy. So genesis was not a cure all for death the way Khan's blood was. Oh Khan still comes from before the time line was altered so how come he had a death cure in his blood but the old Khan didn't?