Does it have to be a binary choice – that in training you have to do more or do less to stimulate adaptation? Ultimately the goal is to get progressively better. I find it interesting that the default usually seems to be how much more needs to be done. I seldom see the option of doing it better.
In my experience, more often it is about better. Instead of a search for more and defaulting to volume as a stimulus for adaptation, I implore you to think about what can be done better. Emphasize quality and recognize that quality is a measure of perfect. Look carefully at your training programs and look for the areas that can be done better. If you are training for 90 minutes are your athletes getting 90 minutes better or are, they just getting tired? I think you find that less is more.