You can say that Guts is like a child but that argument is not perfect as other people as children had many aspirations and dreams but Guts didn't have them. If he wants to have one now, let him live.
Why does Griffith look very mad though? I mean he would still be able to achieve his dreams even if Guts is not around, also there would be no more wars that could possibly happen so there's no need to look over manpower right?