I assumed you were coming at easy run pace and volume from that perspective, and that’s totally reasonable. I’d agree that if a runner is aerobically fit enough that 5km pace plus 2 minutes puts them around 65-70% MaxHr, then great, that seems we are saying the same thing but from a different description. However, if 5km pace plus 2 minutes has them pressing up on 75% MaxHr, I don’t see how backing off to a lower effort and maintaining or even being able to slightly increase volume as a result of that, knowing what research and experience suggests, is suddenly not beneficial?
Maybe drawing on the performance modeling approach, whether Training Peaks or Banister, that Sirpoc and Letsdoit! utilize. If the aerobic stimulus of an easy run is a function of speed and duration, whatever the actual function is that quantifies the stimulus, there would be a range of inputs that could produce the same output value. The idea being that your body recognizes that resulting stress and not that it was 45 minutes at 7:30 pace or 60 minutes at 8:30 pace.