I'd give slide film until the end of the decade. C-41 will probably continue on longer for maybe another decade or two. B&W I could still see being around in 50+ years. Of course, I'm just talking about wide commercial access by big companies like Fuji, Kodak, and Ilford. I think there will be people who create their own emulsions or small, boutique operations well into the future. I get the horse-back riding analogy, but I think oil paint is a better example to use.