If the sport comes down to a level where privateers can afford to run a WRC season, the costs will equally be low enough for manufacturers to have a program for relatively negligible cost. Especially if there's presumably still a market for selling cars to privateers in your scenario. In your doomsday scenario, Formula 1 could be decimated very quickly, but rallying could weather the same storm, as it can theoretically downsize costs very quickly while maintaining a largely similar product.