‘Aerobic training’ is certainly not the only and perhaps not the best method to achieve the referenced adaptations.
I would agree that by rigidly and dogmatically classifying training practices as explicitly and exclusively performing specific functions and yielding specific adaptations, we close off many of our options and limit our understanding about what is really going on.
Perhaps compromise was a poor word choice, and in this context would be sub-optimal, whatever the degree. I would agree that any loss in speed ‘significant’ or not, would certainly not be optimal practice, nor would settling for such a situation. This is an excellent distinction that you make.
However, as performance is often the aggregate of many abilities, particularly the 400m, must a reduced rate of gain for any individual component in the short term be tolerated such that the summation of these components (i.e. performance) be allowed to progress over the long term?
What, if any, reduction in rate of gain, not performance level, could be considered tolerable?