I mean, it´s just an API, a software, which games will be programmed to take advantage of. It should be available at least for the 7000 series of AMD gpus. It should be a matter of updating drivers for older GPUs, I believe. If Mantle is just an api that accesses certain hardware resources directly, skipping on DirectX or OpenGL, why wouldn´t it work with older series of gpus.
The only reason I see is that AMD wants to resell us their rehashed GPUs. It´s just a money grabbing move.