Sperm produced by a man deteriorates with his age. What that means is that this man’s 40-year old sperm is likely to be of better quality (fewer abnormalities) than what the man produces currently.
The question I posed was not whether or not the Japanese man chose to use the 40-year old sperm; but whether or not it was necessary - NECESSARY - to fulfill his goal of fathering his own progeny. Nothing in the article says it was necessary.
As to his current sperm, at his age, being less viable than his frozen sperm - current in-vitro fertilization clinics are already expert at analyzing and selecting the most viable sperm sample from the material the man provides.
No. The choice for the 40-year-old sperm was not about simply assuring he had his own descendents; it WAS, primarily I imagine, about being part of a larger story.