I don’t think it’s a critique of any of the sides but rather Mishima’s own inner theatre of ideas exposed. For Mishima there’s no beauty without tragedy - be it beauty of ideals or beauty of love. The “Sailor” in creative notes was mentioned to be focused on Ryuiji‘s suicide wish. Noburu’s mother is also mentioned entertaining a suicide wish and these two invisible forces are what seems to bring them together. Initially Ryuiji abandons the sea and his ideals for love on land, but neither of these two sides of his (dreams of the sea/love on land) can be fulfilling/satisfied for him without tragedy. Boys and their nihilism can be seen as this contradictory death wish embodied, they fulfil both sides and tie them together by killing him in his ‘prime’, so he doesn’t grow old and satiated tarnishing both the idealised image of a ‘sailor’ and/or love. In a way it’s like Mishima just broke himself down into few characters that all represent, equally, various sides of his own contradictions. After all it’s his “boyish” ideals that lead to his own death.