I can't help but wonder as well if all the population estimates of the original Plains Bison were vastly overblown. I keep reading 60 million head in articles critical of the human causation of the extinction, but most other sources seem less than half that. Wikipedia's reference material suggests 25 million, granted no solid number but none of these are. I wouldn't be surprised if the number was something even lower, like 10 million or less. There were no scientific herd counts occurring, just a bunch of guesses sometimes more than a century later. It is easy to imagine how guesses could be blown even ten times the actual number- just like the hunters estimating their kills, proved greatly exaggerated, and actually anything counted by humans really. We're terrible at it!
I can easily see how a far smaller initial population than estimated could have been destroyed by wanton shooting from railways disrupting migration, unmanaged hunting / killing, domestic cattle diseases, and settling of the plains. Like the Grizzly Bison doesn't do well around people in the wild.
I know I am beating the David Thomspon drum, but there is a lot of first hand information in what he wrote. In 1787 he gives an account of a smallpox epidemic, the odd thing he notes is that the animals also diminished.
"For three and twenty days we marched over fine grounds looking for the Indians without seeing any other animals than a chance Bull Bison,"
"It is justly said, that as Mankind decrease, the Beasts of the earth increase, but in this calamity the natives saw all decrease but the Bears."
"The Bisons are vagrant, wandering from place to place over the great Plains, but the Moose and other Deer are supposed to keep within a range of ground, which they do not willingly leave, but all were much lessened in number."
"All the Wolves and Dogs that fed on the bodies of those that died of the Small Pox lost their hair especially on the sides and belly, and even for six years after many Wolves were found in this condition and their furr useless. The Dogs were mostly killed."
"it was noted by the Traders and Natives, that at the death of the latter, and there being thus reduced to a small number, the numerous herds of Bison and Deer also disappeared both in the Woods and in the Plains, and the Indians about Cumberland House declared the same of the Moose,"
certainly sounds like another disease(s) affected the animals (except bears!) at the same time.