Fuel economy should be a selling point forever. The gas prices are low due to world petrol consumption. Once the Asians start consuming more world prices will increase. Only fools are going out and buying gas guzzlers.
Good point on production. It would cost less just to use the same engine. I mean its just a tuned down version of the Abarth. Just tune that baby up. Well the turbo on the regular 500 might be smaller.
I agree that fuel economy should be important but people have short term memory unfortunately. I currently own a 2013 500T with the detuned Abarth turbo. Everything on the engine is exactly the same as the Abarth. The only thing unique is the ECU, with detuned max boost pressure. Boost appears to be limited to 15psi max compared to 18-21 on the Abarth. Removing the muffler and adding an intake has brought me close to Abarth levels of power. With a chip, I'll blow a stock one away or be on the same level as a modded Abarth. At the time, I was trying to save money but the main reasons I didn't get the Abarth was the car was going to be my delivery car so I wanted a softer in town ride and the ability to add a spare tire. The Abarth's exhaust design negates the ability to add the under car spare. That was a no starter for me with all the screws and nails I catch in my tires.
Side note: you only lose about 2-4mpg with the turbo unless you run it at max pressure all the time. Before my turbo, I owned a regular 500 cabrio. I averaged 35-37 mpg with that car. With the turbo, I average 32-34 even with a slightly shorter final drive.