These are always the worst-case scenarios, the result if nothing is done to mitigate the disease. In every case, measures were taken to mitigate which resulted in the actual numbers of deaths falling below the projected worst-case scenario numbers. For example, in 2009 during the H1N1 pandemic, vaccine companies immediately began work on developing a vaccine--which is straightforward with flu, since they only have to tweak existing flu vaccines. And people started taking other precautions, for example placing disinfectant wipes in gyms and hand sanitizer dispensers in public places. The 2009 pandemic influenza virus is still circulating, still causing higher than average flu deaths every year.
For BSE, entire herds of cows were culled and testing was implemented in slaughterhouses. Currently, if a single cow comes up positive in a slaughterhouse, the production line is halted for sterilization of all equipment, the entire meat production for that day is destroyed, and the origins of the infected cow are traced back so that the source of BSE can be identified and destroyed.
Never assume that because the actual number of cases/deaths came in lower than predicted that it happened spontaneously. It did not. It took a lot of behind-the-scenes work to make it happen.
No, the extreme numbers are not always the worst-case scenarios.
Coronavirus Could Kill 200,000 in U.S. in Best Case Scenario ... .. 5 days ago - Projections of 100,000 to 200,000 American deaths is a best case scenario ...
https://fivethirtyeight.com/features/best-case-and-worst-case-coronavirus-forecasts-are-very-far-apart/: On Tuesday, Drs. Anthony Fauci and Deborah Birx — members of the White House Coronavirus Task Force — estimated that COVID-19 could kill 100,000 to 240,000 Americans even if all federal guidelines are followed closely. They believe there were between 289,000 and 12.8 million infections, with 1.1 million being the consensus estimate, implying that the experts think that only about 12 percent of all infections have been reported.