The $100 million was for both movies. Since they were shot back to back. By now, WB has made back at least $50 million from theater sales, digital and physical sales. Plus any uptick in streaming revenue for people that waited to watch it on the MAX app.
I agree that today's young audience just isn't into long westerns or Westerns in general. The numbers for westerns over the last decade are just not that great. We can say that streaming shows that have western themes seem to be doing great (Yellowstone and all of it's spin-offs). So I suppose maybe then WB should look at doing that with Costner's Horizon Saga.
Like I mentioned before, I'm glad Costner was able to get the first one in theaters. But now, I would be A-ok if they put the remaining chapters on streaming (for now) in the form of a series. So he can finish this story his way. Then release a nice 4k box set!