I think it has more to do with the performance aspect of endurance athletes (who actually compete or want to maximize endurance performance).
I don’t think it is depicted as evil as much as it is seen as unnecessary and something that just builds up workload, reducing how much of the “other zones” you can do.
When it comes to physical performance, if you train at both ends of a spectrum, the middle will also improve.
For example, if your lifting includes work in the 1-3 reps range and in the 20-30 reps range, you don’t “need” to train in the 10-15 range, it will improve as much as if you trained it directly.
For endurance performance, the longer, zone 2 (long, low intensity), runs will build an aerobic base. It improves mitochondrial function (which makes you more efficient at producing energy from fat).
Zone 4 and 5 (higher intensity, shorter duration), improves VO2 max, the capacity to use lactate to produce glucose to be used for fuel, clearance of metabolics as well as peak aerobic power and anaerobic capacity.
So yeah, from a performance standpoint, if you only do zone 2 and either zone 4 or 5 your performance will increase across the whole spectrum of endurance events.
The medium intensity (zone 3) benefits is mostly to improve lung capacity and it will, of course, have a positive effect on the capacity of the heart.
The point is that zone 3 (medium) is NOT necessary for performance. That’s why it’s being used less and less. And when it’s being used it is mostly as part of a fartlek workout where you do most of your workout in zone 2, but include short bouts in zone 3.
So it’s NOT evil. It’s just not necessary and volume is going to be better spent in other zones.
For pro-longevity the use of zone 3 is, IMHO, when someone is not physiologically prepared (not in good enough shape) to do zone 4 or 5 work. It’s like a bridge that will help you get fit enough to do the harder zones without trashing yourself completely.