Well, that formula shows that if you make the assumption all else is held equal. The problem as I am pointing out is that all else is not held equal
GDP = C + I + G + (X-M)
You're right as far as you go. The role net exports play in this formula represent a direct drag on US GDP growth; an example of an indirect drag on US GDP can be observed by the effects outsourcing has had on Investment's role in growing US GDP. When major US corporations build factories outside the US, they are also reducing the "I" in the formula.
For twenty years the US and EU ran trade deficits with China of well over $200B per year. By 2012 both developed economies were too weak to sustain China's export dependent growth.
From my perspective, we are currently dealing with a long term structural issue of chronic, global trade imbalances that can not be addressed by continuing to enrich the investor class at the expense of the majority.