Final answer:
The sentence from the excerpt highlighting the retirement of Japanese boatwrights and the consequent loss of their knowledge indicates why the art of traditional Japanese boat building may disappear in the future.
Step-by-step explanation:
The detail that best shapes the idea that the Japanese art of boat building may be lost to future generations is: "Even in Japan, where traditional crafts are revered, this system is too grueling, too much at odds with modern life, to survive. It is no wonder, then, that as a generation of Japanese boatwrights has retired, their knowledge has retired with them."
This sentence provides a direct connection between the rigorous nature of traditional apprenticeships and the contemporary context that makes the survival of these practices uncertain. It underscores that the retirement of skilled boatwrights could lead to a loss of their profound, yet unrecorded, knowledge.