I think there is litlle motivation in athletes in that age to continue in Decathlon.(At least in my country-Greece).
Here is a question for you!
How many 18 years old athletes can learn Pole Vault from scratch?(without getting discouraged?)
It is easier to continue with one or two events where he's more able and it's easier to train...
Plus, how many years until he reaches a decent mark?(about 4.30 to 4.50)