In looking through sources, I'm no longer even convinced that disease was relevant anymore. An encyclopedia article shows a decline from about 15 million in 1865 to 7 million in 1873 - roughly 1 million per year.[1] This sounds like a gradual decline to me. And with some math Buffalo Bill famously killed 4000 over two years. Assuming the average hunter only did a tenth of that, it would only take 5000 hunters over 20 years to get the job done (roughly).
Furthermore, the National Park Service, part of the Department of the Interior, quotes the Secretary of the Interior of 1873 stating that "[t]he civilization of the Indian is impossible while buffalo remain on the plains”.[2] In fact the DOI secretary in 2023 said verbatim that "bison were nearly driven to extinction through uncontrolled hunting and a U.S. policy of eradication tied to intentional harm against and control of Tribes". [3]
This is not hard to believe at al. There were centuries of war between the natives and the settlers. Presidential campaigns slogans focused on defeats over the Indians[4]. The U.S. absolutely hated the Natives from the start. One of the cited grievances in the Declaration of Independence is the fact that Britain would deal with the Natives.
It is both plausible and proven that extermination was the goal. If you still don't believe this here is the most detailed timeline I've ever seen on the subject from the US Fish and Wildlife service. [5]
[1] https://www.encyclopedia.com/history/encyclopedias-almanacs-...
[2] https://www.nps.gov/articles/000/what-happened-to-the-bison....
[3] https://www.doi.gov/pressreleases/interior-department-announ...
[4] https://en.wikipedia.org/wiki/Battle_of_Tippecanoe
[5] https://web.archive.org/web/20200210033215/https://www.fws.g...