[angrysaki]

There hasn't been much research, but Stephen Seiler has done some studies with polarized training down to 5-8 hour per week athletes which works well in those situations.

https://scientifictriathlon.com/tts177/#tab-con-16

I think the reasoning is that there are 2 main signaling pathways for endurance adaptations, one from high intensity and one from low intensity and you want to stimulate both for maximal adaptation.

The other part is that if you have more time to train, you simply can't do more and more HIIT without burning out. There are pro cyclists who did very little intensity until race season and just do lots of low intensity.

From what I remember, even elite rowers generally follow an 80/20 polarized model and their events are in the 6-7 minute range.

[crimsonalucard]

A lot of training regimes are used because they are effective but not scientifically proven to be the best possible training method. I'm guessing that the examples you are talking about used training methods that were effective but not the scientifically verified to be optimal. It's possible these athletes may have hit their maximum potential but had to do way more training than necessary.

It is scientifically verified that HIIT counterintuitively effects the aerobic signaling pathway at a greater rate than aerobic exercise itself. My other comment in this thread cites tons of good sources:

https://news.ycombinator.com/item?id=22157311