It's not that simple. Crop rotation must be part of the equation, so twice as much area must be dedicated to the task. The choice of alternative crop to address soil depletion is a critical (beef production?) component of the long term solution.
Here's a question. How much current agricultural crop production is subsidized (i.e., how many acres of crops are grown currently that would not be profitable if they weren't subsidized)?
If switchgrass production was profitable, all the acreage that isn't profitable could be converted to switchgrass. I don't have any numbers, so I can't answer my question.