First of all, Firewire was never able to transfer arbitrary PCI traffic, thus you couldn't use it to attach external PCI/PCIe devices to your portable device. When PCIe "extender" solutions became available, they were expensive and bulky. The connectors were huge, and the cable thick, and sometimes it would just refuse to work in a particular setup. Thunderbolt provides this kind of functionality on a manageable, off-the-shelf interconnect that you can buy in nearby Walmart. A brand name thunderbolt single x16 PCIe card cage runs about $500, and you can buy off-brand ones for half that. This lets you pull off stunts like adding two graphics cards to your laptop. I'd say calling it a "bigger flop than firewire" is borderline trolling.
Connecting "directly" to PCIe for expansion/extension purposes is setting the clock 10 years back - if you have any PCIe to attach to begin with. Fewer and fewer laptops have expressCard slots, and some high end laptops rightfully (IMHO) got rid of them. I don't really miss expressCard on MacBooks. Thunderbolt is much easier to deal with.