E85 has proven to be a replacement for gasoline, may require some work in some cars, but is a very direct swap with tuning. CNG requires much more work, almost reengineering, so I don't see that.
If you take GM (obviously I know mostly) for example, a large percentage of their cars are flex, even more require a simple reflash. The spin of ethanol being renewable gives people with old cars an option. Not to mention most of their cars could easily be self-driving, and I am willing to bet GM has prepared for the future.