Erm, I don't know if one can really say that heart rate is proxy measure of VO2 max. The most readily contradictory example I can think is the fact that regular training will raise a person's VO2 max and the heart rate will remain the same. As a person's V02 max improves, the heart rate doesn't get higher, especially as the muscle cells develop more mitochondria and more capillaries are developed to help deliver oxygen to the muscles.
My own view has come to see heart rate as a useful measuring gauge of effort. The biggest initial difficulty with that measuring gauge is to calibrate it, and there are 2 important measures to take in order to do so -- heart rate max and resting heart rate. HRM can be estimated by age and activity formulas but it's probably better to do some sort of actual physical test. Resting heart rate can be taken just by measuring it shortly after waking up.
Now my own experience regarding easy effort is that it is possible to fool ourselves. I began running again last year at what felt fairly easy, I could converse and all that. But when I did get the Garmin last Christmas and began using it, I often found that what I though was my easy pace was usually somewhere in the low 140s. With the numbers and data I now have, I know that my truly easy pace needs to be 135 or 136 bpm or less. The toughest thing, especially early on, about running under that heart rate was that I felt damn slow and my ego got chafed and irritated. But that pace has been improving and now I feel better that I am truly running at easy effort.
That doesn't mean I don't think the running I did earlier on where my heart rate was somewhere between 140 to 148 and in the sort of in between zone between easy pace and threshold/tempo pace was useless. I think it's useful and obviously lots of people do lots of running in it. The basic problem is that it may take a little more out of you than you realize and that impedes the effectiveness of the harder running, along with the fact that because it is putting a little extra stress on you, you won't get as full of recovery and enhance the risk of injury or even illness maybe. So that's why it might be very effective for more people to learn to run truly easy, below 70% of their heart rate reserve and develop an area of cardio fitness that's being missed or squandered in a sense.
I definitely agree with you that the more miles you run, the more you need to do them easy. That's what I'm working with right now. But it is also possible for a person to get faster by doing 3 or maybe even 4 harder workouts per week, example hill running, tempo, long run, and improve their fitness. The key is recovery, so take a day in between those runs and recover. That's the basic program of sorts that
Fell Runner uses, where he has pretty much dumped out doing easy runs and runs 3 or 4x a week.