AGP vs PCI

I know this ia an age old argument. So, I have heard that AGP will be gradually phased out in favour of PCI. Motherboard manufacturers will ecentually not have AGP slots. It is said that there is better bandwidth in PCI slots than AGP.

Why won’t they just advance technology to make 16x x AGP slots? They have made the advance from 2x to 4x to 8x. How hard is it to make 16x?