It pretty much has already happened. Most cars that offer both versions are very similar in terms of mileage for automatics and manuals - the difference is usually only like 1 or 2 mpg, with the automatics actually coming out on top at times because the car makers can add more gears. And realistically, most drivers aren't as good at using manual transmissions as they like to think they are, which also does a lot to cancel out the theoretical improvements a manual transmission offers.