> This doesn't mention the function slot of the symbol, so one could perhaps argue that therefore the function slot is not meant to be a constant variable

Exactly

> but one could equally well argue the opposite

No, as variables and functions are different parts and there's a more general rule about functions applied to keywords.

Your argument seems to be based on a notion that keywords are something unique and special in CL. They are, but only to the extent that is specified in this section. Otherwise, they are the same as other symbols.

To me, Lisp is not about "forbidden all that's not explicitly allowed", but about "allowed all that's not explicitly forbidden" mentality. And it's, actually, an important trait of the language that makes me value it more than others. So, sorry, I value your opinion, but I think that such things as := need to exist if only to broaden the horizons of people with such opinions :)