Note that AGP was initially basically just PCI 2.1, with the bus conflict resolution system ripped out, the connector flipped around and a few minimal tweaks. It could, in principle have been used for something other than a video card.
The crucial part about it was that it was a dedicated link to one device. AGP was initially created not because PCI bandwidth was running out, but because PCI is a shared bus, and the kind of transfers video adapters liked to do played havoc with the system that negotiated who had right of way, resulting in all kinds of problems when other cards had to wait for their turn for much longer than their driver developers expected.
The fact that it only ever connected one device to the host made it much easier to evolve, as future cards and hosts could just negotiate to do something different than what AGP 1.0 defined, if they found they both supported a faster version. When bandwidth demand rapidly rose with ever faster 3d accelerators, this was very beneficial.
The crucial part about it was that it was a dedicated link to one device. AGP was initially created not because PCI bandwidth was running out, but because PCI is a shared bus, and the kind of transfers video adapters liked to do played havoc with the system that negotiated who had right of way, resulting in all kinds of problems when other cards had to wait for their turn for much longer than their driver developers expected.
The fact that it only ever connected one device to the host made it much easier to evolve, as future cards and hosts could just negotiate to do something different than what AGP 1.0 defined, if they found they both supported a faster version. When bandwidth demand rapidly rose with ever faster 3d accelerators, this was very beneficial.