Missing the AF_BUS

Can you give me an example of who is burdened by what exactly in this case?

> The reason that I would avoid adding multiple address families for IPC is
> that someone would want a mix of features from one and features from
> another (e.g. multicast and fd passing from AF_INET and AF_UNIX). So
> would we add yet another one that does both?

That seems like a speculative reason to reject existing and well established software patterns like d-bus, that are correctly leveraging a well established extension mechanism for adding new protocol families. Again, if it wasn't meant to be extended, then why have protocol families at all? Why was the 'sk is first member of the struct' pattern so well thought out from the beginning? It was done this way to provide ways for the mechanism to grow and evolve.