Here's what we know:
1. If you run zero miles per week, you're not going get that fast.
2. The more miles you run, the faster you'll be, up to a point i.e. diminishing returns, which will be different for everyone.
3. There are low responders and high responders to physical stimulus(mileage is a stimulus).
4. How you do the mileage matters. Is it all over hilly/mountainous terrain? How much tempo work? Speed? Etc.
5. Everyone's sweet spot will be different. Look at Bill Bowerman. He had Kenny Moore run low mileage, even though he was marathoner. Yet, had some of his milers and 5000 guys running north of hundred miles a week. And the frequency of workouts was different for Kenny Moore, his training schedule was adapted to a 9 day schedule instead of 7 day schedule. So how much mileage(and frequency of the workouts) you should do DEPENDS on the person.
So, high vs low mileage? This is a false dichotomy. We shouldn't focus on the mileage. We should focus on what works best for you. Some will run sub 13:20 on 40-50 MPW and others it will take 100-120+ MPW to do that. Some thrive on high mileage and others thrive on low mileage. So don't get all caught up on high vs low, find what works best for you.