As Balthazar says, modded cars are a tiny percentage vs. overall sales.
I think that you are giving way too much creedence to import performance being the wave of the future.
The fact is that performance crosses all segments and configurations these days. And, as they have done for most of their history), GM is producing the most powerful entries in many segments.
Witness the Cobalt SS.
That however, does not obviate the demand for RWD or V8 power. Many stuck with high winding 4s or 6s would have a V8 performance car if they could.
I will agree with you that the V6 is critical to Camaro's success- now and in the future. But GM has really upped the ante with the 5th gen V6 car - it is quite simply the first serious V6 Camaro ever built. All previous gen V6s were mere posers. This v6 is a serious performance car, not an afterthought.
Right now is exactly the worst time to bail on RWD and V8s. It is time to apply them carefully and well, but they are the (sometimes secret) passion of American performance indentity and desire. Such cars must be done well to carry forward a valid identity for the domestics when it comes to performance. The "lesser" entries in FWD and RWD form with V6 and 4 - banger power must not only be equally good in their segments as their V8 brethren, they must also distill that very heritage down to their own corner of the playground. They must be seen as distinctively American performers if they hope for any traction in the market. That alone is reason enough for the domestics to keep their "traditional" performance entries, not only around, but excellent.