Even NVIDIA isn't crazy enough to try to get everyone to use the same ISA again; they're trying to get you to buy into their compiler chain, not into the architecture of a particular generation of GPU.

Is PTX really ISA? It's also virtual, but very limited ... Unlike PTX, FSAIL will work on both CPUs and GPUs and even will allow system calls from discrete GPUs.

PTX seems quite reasonable to me, but I don't think it's really intended to be a programmer-facing abstraction. NVIDIA wants you to target CUDA, and they'll (try to) ensure that CUDA has sufficient performance advantages to make it worth your while to do so.