Yes, we are in agreement. My point was that developing oxidative abilities allows for extended glycolytic contributions rather than diminished.
From an energetic standpoint, I believe the benefits of aerobic training for the 400m athlete are primarily two-fold-
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Increasing the capillary network to aid in the oxidation and removal of waste products, as you mentioned, thus allowing for glycolysis to continue to function at a high rate.
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Improve the maximal rate of ATP synthesis through oxidative processes such that when glycolytic ATP production does inevitably slow, the drop-off is less severe. All things being equal, the higher the maximal rate of oxidative ATP synthesis, the less the drop-off.
Of course, the trick is to develop these adaptations without significantly compromising improvements in speed.
In regards to knowing the anaerobic threshold, if valid and reliable testing can easily be performed, more information as to the state of the organism would almost always be of benefit. I’m just not sure if we should specifically aim to improve the anaerobic threshold for this type of athlete.