Would all the tribes have done it? No. That's the issue here. There was no unified agreement amongst the various indigenous peoples of North America. There were thousands of languages, religions and cultures.

So would the Blackfeet, who basically believed that all bison must die for the good of their people? Probably they would have.

Would the Crow, who hunted more sustainably? Probably not.

So I admit I was being far too general when I said 'they.' I should have specified that certain groups would have been fine with wiping out buffalo and others were more sustainable hunters.

Oddly enough, De Soto, who spent a lot of time wandering around what is now the Southeastern U.S., didn't report buffalo there. So it's also unclear how widely-ranged they were in the first place. There's an author named Charles C. Mann who wrote a well-regarded book about pre-Columbian contact called "1491: New Revelations of the Americas Before Columbus" and he posits that the vast buffalo herds later reported in the Great Plains were actually the result of the mass die-off of indigenous peoples through European diseases returning the plains to grassland when they had previously been farmed. So it could be that there were also far less buffalo in the Americas before European contact (although most certainly more than today).

And no, I would not deny the reason why the mass kill-off later happened. It was one of the many genocides the United States perpetrated against the indigenous peoples of the Americas.