I don't think that's the whole story, though. With some advances in storage, PCI-e is not far from being a bandwidth bottleneck in even consumer PCs (thinking about NVMe as an example.)
Even there the CPU can be the bottleneck. I have a Samsung 960 Pro that's theoretically capable of 3GB/s reads but when you use disk encryption even with AES-NI the processor can only do ~2GB/s.
What are the current maximum bandwidth speeds used by PCIe solid state memory? I'm not aware of any that go over 4GB/s, let alone the 16GB/s of PCIe 3.0.
