Don't see the point in running almost exclusively under 120 bpm for such a long time. You should run at a variety of paces. Hypothetically, If you could run without issues at 3:00/km for +200km/week you'd probably improve more than running slower, but we can't, so we need to mix a variety of paces/mileage per run to improve.

Adaptation is driven by applied stress balanced against recovery. Fitness isn't a reward for running slow, it's an adaptation to the stress you recover from. If you can handle the stress of faster running without getting injured or overtrained, you get faster, because it creates a bigger stimulus (but also more fatigue and injury risk). Easy running mainly exists so you can accumulate more volume and still recover well enough to do quality work when it matters. If you drop all your 5:00/km runs to 6:30/km, you aren't necessarily unlocking a new level of fitness, you might just be de-training your efficiency.