> 4)ports are different, protocol mismatch : won't work. Adapters might exist but those won't work either.
Why would adapters necessarily not work in this case? It looks like you're claiming that translating one protocol to another is always impossible, but that can't actually be the idea... right?
Most of these kind of adapters (usb-c/usb-a, dvi-d/hdmi) don’t actually do any protocol conversion, they merely take advantage of fallback/cross-compatibility that the endpoints actually implement and rearrange some wires. Adapter that need to do something more complicated (like dvi-d to vga) have to be powered. And in some cases like USB 3 to thunderbolt or DisplayPort it’s simply impossible for the host controller to provide the necessary IO. Thunderbolt expects to provide a full PCIe interface to the CPU that doesn’t fit within how USB 3 controllers communicate with your CPU. DisplayPort requires a connection to your graphics adapter, which is also not under the USB 3 controller’s purview.
No, in case 3 there's a protocol match. In case 4 there isn't. I'm asking why an adapter is supposedly unable to deal with a protocol mismatch. Dealing with mismatch is the whole idea of an "adapter".
If an adapter exists we can categorize it as case 3 for the sake of this discussion. I think you're needlessly hung up on the word protocol when that really doesn't matter.
Why would adapters necessarily not work in this case? It looks like you're claiming that translating one protocol to another is always impossible, but that can't actually be the idea... right?