If anything I would think his being a prodigy has taken a toll. His body has had to work far harder for longer than for most athletes his age. Andre Agassi, the great tennis player, said when the end comes in a career it comes quickly. I believe we are unfortunately seeing that with Ingebrigtsen. The recurrent injuries will end his career. He can never regain what he was.
He will have a second act I think. Interesting as always, we can't fool mother nature. Every time there is a supposed shift in training philosophy and the promises that come with it, the body isn't fooled. All this junk about double threshold being easier on the body sounded right when he was young and injury free. Seems like no matter what we do, nature finds a way. Stress is stress.
East Africans have genetic advantage born at altitude blah blah blah... well the world caught up quickly. Mostly euros and americans in 5k final. Euro wins 10k. Seems like everything comes back to square one.
Its not "junk".
There are a few different thoughts. That treshold training is easier on the body than hard anaerobic training, this is true. Its sustainable. Way easier to do year after year etc.
But, is it easier to run 180km a week, controlled with double t, than 110km (Kerr-ish) with slightly harder intervals and (maybe) more anerobic and hard stuff? Who knows, but thats not the argument.
Double t is just a way to keep athletes from overdoing it when theyre on high volume. Thats it. Of course thats easier, especially as there are few 30km long runs etc and way more doubles which is way easier on the body, but admittedly takes a bit more time to get progress from.