Couple what laborsmith said, with the fact that nearly all other manufacturers of subcompact cars have updated styling/redesigns since the sonic came out in 2011/2012.
I think the success of the Versa show that people in the entry level subcompact segment are mainly interested in the bottom dollar price + mpg. Look at how the Mazda 2 failed miserably when it didn't meet these criteria.
If they want sonic sales to be competitive, they need a new face and rear, and to come out with an eco model next year gaining 1-2 highway mpg (similar to what they did with the cruze), and throw more money at marketing. But are these costs worth the increase in sales in a slow subcompact market? I think what GM is saying to us is no, not until gas prices are higher.