Sure, AMD could turn their Promontory 21 into a universal expander chip without much effort, if only they wanted. It's not like that nineteen-wire serial interface (??) (what??) is absolutely necessary. The PCIe interface is flexible enough to carry all commands, control information, metadata, sync, whatever is necessary for a port expander/PCIe switch to function.
It's not about being capable enough, but rather about PCIe not supporting the extra control signals that are required for some interfaces to stay synced properly with the rest of thes system.
That said, I don't understand why this has a more complex connector that the Thunderbolt cards, but it's possible that ASRock has to use multiple "slow" standards.
I agree that this seems overly complex and if the chipset was only used for PCIe, it would most likely end up being simpler than this, as it shouldn't require to sync with so many other subsystems.
Technically it ends up being a four to eight PCIe 4.0 bridge/switch, plus four PCIe 3.0 lanes if it was used as a "universal" PCIe expansion solution.
would like to see back plain boards making a comback like they used in the 1990...
I think you want some version of the PICMG form factor for that, which is sort of a continuation of the AT form factor for industrial use, as in your picture.