I have seen "non feeding ground " elk mega herds in person in three states. Oregon, Utah and Wyoming.
In Utah every year I hunted a specific ranch, had the land owner tag for that ranch, I coulld not hunt anywhere else, even on the adjacent National Forest.
The last time I hunted there it was unseasonably warm and the elk did not come down. They stayed up high on a sister ranch, same extended family, but required a different land owner tag.
I looked at about 1,200 elk, all bunched together. I could not shoot one there. just about 75 miles to the east was the center of Utah's growing CWD crisis. It is only a matter of time. Mega herds are so vunerable to diseases like CDW and hoof rot too.
Utah's ideal elk population is about 67,000 elk. they have about 81,000. Mule deer are way down. There is only so much feed and there is a huge drought there. The math is the math.
In a narural suituation what breaks up mega herds are large preadtors, wolves especially.