It certainly seems ironic that the company that beat Intel at its own compatibility game with x86-64 would abandon compatibility with today's market leader.
The situation is a bit different: AMD got its foot in the door with the x86 market because IBM back in the early 1980s forced Intel to license the technology so AMD could act as a second source of CPUs. In the GPU market, ATI (later bought by AMD) and nVidia emerged as the market leaders after the other 3D graphics pioneers (3Dfx) gave up - but their GPUs were never compatible in the first place, and if AMD tried to make them compatible, nVidia could sue the hell out of them...
