Yes, well, sometimes there are both de-facto standards and de-jure standards in circulation.
XHTML 2.0 comes to mind.
Tactically, creating a backward-compatible successor to CUDA (a superset, essentially) would have the advantage (for AMD, that is) that NVIDIA can't decide to un-implement the existing CUDA support in their products in order to to spike AMD's efforts.
Then again, there is a lot of IP sloshing around in this space, so the specific tactic used by AMD may have to be something sneakier and subtler.
Anyway, this is all just one of the hypothetical ways AMD could try to unseat NVIDIA, there are others.
I thought OpenCL was the standard in this space.