I think you've nailed it. Hall has years and years of big miles behind him and I don't think he'd have been as fast as he has without those miles. But after a while you don't need as much mileage as you once did and may be better off on less.
After about six years at 100-150 weeks I had almost two years with no PRs and dropped to 75-90. I got one last stretch of PRs from that. If you followed Ron Hill's career, you saw that he did 120-130 mile weeks when he was prepping for marathons from the mid 60s to mid 70s. Then he dropped his average to around 65 per week with high stretches of 90 or so and ran some of his better times, though he never got under 2:12 then.