AGP will die because PCIe is pushed by the industry, its cheaper and has some potential to implement faster chipsets.
Currently, as everyone has pointed out, it does not provide and technical advantage but the market made the transition and its hard to get AGP graphic boards of the newest generation. The next generation of graphic cards will be PCIe only... no more AGP.
Expansion Cards where thought to transit to PCIe too but that didnt work so far, to many people use their current PCI cards. Motherboard makers already have cut down on PCIe (x1 and x2)slots on the boards cause no one uses them.