Motion picture stock is not really tied to still film. It won't keep the emulsions you use around, nor will it kill them if it goes away. Different stock. Different emulsion. Different process. 35mm still film will stand or fall on its own. Some of the ongoing R&D for MP benefits the still camera emulsions downstream. Kodak dumped B&W paper, and still continues to make volumes of color paper, so shouldn't the color paper have kept black and white around? Obviously the answer is no, and thats about how close MP emulsions are tied to still emulsions (from the pointy end). Only Kodak and Fuji are even in the MP business, yet a plethora of other companies, especially B&W are still in business. If demand for the product is there they will continue to make it. If demand falls too low it goes away, however the notion that motion picture keeps the still film used in our cameras around is false, except the contribution it makes to a bottom line ie the overall financial well being of a company, and the volume purchasing of some raw materials. Kodak could stop making still film, and continue manufacturing MP, which is the likeliest scenario, if demand for still emulsions fall. We have already seen this regarding some emulsions. It's a cherished notion, a nice thought that it keeps still film around, but not on the money. The thing that keeps still emulsions around is the demand for it. BTW, motion picture film positively sucks for stills, even when you process it correctly.
While film use has drastically declined, keep in mind that at the peak it was a 13 billion with a B dollar a year industry. Even with the decline there's still some money left in the old horse.