I guess the main reason to why these post 2009 cars has been adding longer and longer wheelbases was because of the restriction in the bargeboard area. You get an advantage in moving the front wheels further away from the sidepods and floor, so the wake has less effect on those areas. And without the bargeboards to help moving the wake outside of the sidepods/floor you could gain an advantage in moving the front wheels further forward.
The increase in floor lenght is also helping to create more DF.
But now with these new regulations, allowing for bigger and more complex bargeboards, why not make a fixed wheelbase. It would be the penalty for every team. They would be much more pleasing to look at, and would be more twitchy to drive, not so much as driving on rails as these cars are today.