Crossfit has two or three specific articles on the various metabolic pathways and how they relate to your hypothesis regarding excessive endurance work. Two of the articles are free, "CF Foundations" and "What is Fitness?". Another is the 06-2003 backissue on metabolic conditioning.
I agree with you for what its worth having boxed for years. Boxing is an anaerobic activity working primarily in the glycolytic pathway(anaerobic) with significant development of the oxidative(aerobic) if training is sufficiently intense.
Boxing requires speed and power, the two very things that are diminished by straight endurance work. By training to develop speed/power, you develop the aerobic capacity specific to the activity itself as well as becoming more proficient/efficient which conserves energy. Your boxer friends would be way more productive using all that aerobics time running sprint intervals instead.