I agree, calling PCI-e a "socket" is technically correct, but not in spirit of the question..
Because a socketed CPU is mainly such that you can freely pair a motherboard and CPU of choice. There is nothing from stopping you putting a Ryzen 3100 into a high-end X570 motherboard with 128GB of RAM, onboard 10Gigabit LAN, extra M.2 carriers, etc.. Does it make sense? I wouldn't say "definitely not": I for one run a dual-core Pentium in a dual Gigabit LAN Motherboard with 24GB RAM, as it serves as my home router, docker server & ZFS storage box.
But then PC applications have a very large dynamic range of I/O, RAM or CPU requirements, in particular if you look at the ratios. How would such component interchangeability compare with a GPU? Usually GPU manufacturers target only a handful use cases: gaming at 1080p, 1440p, 4K, VR, and compute. Usually these use cases scale both in terms of VRAM and FLOPS. E.g. 4K uses more VRAM but also needs a powerful GPU core. Maybe compute is the one exceptional case where in AI you want to load large models and do relatively few calculations on them, or vice versa. But that's a niche IMO
So then, if you want to socket a GPU, do you also want socketed graphics memory? Okay, hypothetically let's say it's possible to build such a system in a compact way... (as GDDR6 VRAM also needs a lot of cooling), I think the winner stories for such upgrade paths are limited. E.g. hypothetically you could swap a RTX2060Super for a RTX3070 and keep the same memory system (both have 256-bit 14Gbps 8GB GDDR6), and get a 50% performance gain. Great! But if you want to upgrade from a 2080Super or 2080Ti to even a 3080 core, you would also need to replace the VRAM. So at the low/mid-end users may benefit the most, but margins are the thinnest at that end. High-end card users may pay the additional extra for socketed systems, but would have the least benefit as virtually everything changes in the high-end GPU generations..
That also brings me to a final point: I wouldn't consider upgrade paths between multiple generations. For example, the 3080 and 3090 are the first cards to use GDDR6X. That makes it very likely that the next generation x60 and x70 cards may also use it (or some "upgraded" version of GDDR6 14Gbps). If you need to keep on swapping VRAM in combination with GPUs to get the most benefit, than that is very different from slow vs fast DDR4 that is very unlikely to severely bottleneck a system if you move it around 3 or 4 of CPU generations.
Any other "platform" support is hypothetical. Like, AM4 boards would support many generations of CPU's.. but then it also does not. Some boards have dropped support because the BIOS wouldn't fit on the onboard FLASH. Even if it would, it's a hassle to research and wait for BIOS updates so you can pair an old board with the latest CPU. Although it's nice to theoretically use a B350 board with the latest 5950X, I think that almost all PC manufacturers only have a working window scope of 2 or 3 generations of products before everything is upgraded again.. And really, I don't upgrade my PC any faster than 2 or 3 generations, so in that case I might as well build a new system (CPU+MB+RAM+GPU).