I think it's easy to oversimplify it to just "double threshold = success" because it's an aspect of training that many are unfamiliar with, and it can seem attractive that the key to running fast is one easy fix in your training schedule. I remember tons of people had similair opinions about CV pace and tinman a few years ago ...
For one, Jakob isn't the first athlete to do double thresholds, I believe that Maris Bakken (13:06) did it first, and there were surely others before him. I'd attribute his success to: Jakob 1. is extremely talented 2. grew up in an optimal high performance training environment w/ 2 olympic athlete brothers and 3. has a an amazing coach. So Gjert's training philosophy (not just double threshold days! the whole thing ...) is just one aspect of his success.
Next, I don't really know where you got the idea that a lot of western runners used to train medium effort quality and low/moderate mileage. What time period are you thinking of? The two main paradigms for mid-d/distance training are high mileage (90 mpw+), slightly less intense workouts (2 workouts + LR), which most americans seem to do or lower mileage (40 mpw) and very high intensity (makhloufi, cheruyiot etc...). No pro would really be doing lower intensity and low/mod mileage nowadays.
Who has copied Jakob and achieved similair success? The next best western runner is Josh Kerr, and I don't think that his training is close to Jakob's. Same for other top western 1500m runners: Kerr, McSweyn, Katir, Hocker, Mechaal, Hoare etc.. I know Berglund seems to have copied him, but I wouldn't put him as the main reason behind the resurgence of western 1500 runners ...