GeForce 8 series makes sense to cut AGP off because by that time, no gamer will still be using AGP who wants to have a high-end system, and it would be impractical because systems with AGP will be too old to be worth upgrading anyways. IMO, they should have produced the GeForce 7 series in AGP because at the time the 7800GT/GTX was released, there were still a lot of people using AGP, people who had excellent systems except needed a graphics card upgrade.
Nvidia though is not going to produce AGP 7800GTXs, because their whole theme this generation is promotion of SLI, and producing AGP 7800GTXs would give people reasons to hang on to their AGP motherboards therefore just buy one card instead of possibly two.