Threshold stuff was pretty big 25-30 years ago. For some coaches it was really all the physiology they bothered to learn.
The whole shebang was hooked on the idea that oxygen delivery is all you really needed to be a successful distance runner and that blood lactate was just bad to the bone. Then the threshold turned out not be anaerobic or even a threshold. Lactate was found to be a useful metabolic fuel actually required to augment VO2 uptake at higher levels. That famed Conconi HR deflection point? Not really a deflection point at all. And of course it turned out that overtrained distance runners show blunted lactate responses. Less lactate, slower times. For top performance you want high lactate values at any given RPE. Later on, the famed 4mmol fixed blood lactate value turned out to be neither steady state or 4mmol. Then Noakes presented data showing the true best predictor of endurance performance from a treadmill test was the speed at finish of the test, this a diplomatic way of saying most everything else believed up to then should be scrapped or the very least, reconsidered.
The final straw was the realization that the very best long distance runners have muscle fiber characteristics much more in line with those of middle distance athletes. Those remarkable type I oxidative fibers turned out to be not so great after all. Moreover,many different combinations of intensity and duration provide more or less the same sort of mitochondrial changes. And especially high volume training is optional. It isn't all about the mitochondria either. You can slow a runner down quite effectively by fiddling with levels of some rather useful neurotransmitters.
This brings us back to your training. Is running "threshold" necessary to become a great runner? No. Is interval training sub-optimal? No, not really. Ultimately it's the mix that matters and most importantly, the realization that no two runners are made exactly the same; thus, no two runners need train the same.
We honor the great physiologists and coaches of the past by challenging their thinking rather than accepting their ideas as sacrosanct. Anyway, the bottom line here is that if you want to reach your potential as a distance runner you will train at a wide range of running velocities, and your top speed will end up being just about as important as your ability to maintain speed.