I think I can answer this: tell me what happened when AMD dropped the Radeon 8500 from the product list back in 2006? Shortly after that release Fglrx starting having feature creep. AIGLX was added. This year, Crossfire was added (for HD 4x00). Catalyst A.I. has been added.Will this action of discontinuing support for older hardware result in an fglrx that is much smaller, substantially more stable, and better able to cope with the kernel release cycle?
So yes, the idea is that cutting down the driver matrix will make for a leaner, meaning, easier to maintain package.
Also, it could mean performance boosts in 3D. Currently the 3D Fglrx core, as indicated, is largely based on the architecture from the older R300 ArtX drivers. Moving to a 3D core that STARTS at Shader Model 3.0 could mean even MORE performance can be squeezed out of the RadeonHD series.
Whether or not that's actually what happens remains to be seen. For me, I'd say give AMD time to get the details worked out.