My off-hand guess as to the demise of 'kith' as a common word is that it's a little hard to say (and to distinguish from lisping -- and why isn't 'lisping' spelled 'lithping?'), and is a little less specific in meaning. I know who my kin are. I would have trouble defining any given person as 'kith' or not; I would have to think how many degrees of separation there are between said person and myself.

I suppose I would call someone 'kith' if I can recall their names, unlike some of my second cousins twice removed (and I'm getting a court order if a thrice-removal is needed). So if someone is on my Facebook page as 'friended,' he/she is kith, but if they're my aunt's daughter's son's wife, she's kin of a sort.

So what about the participants in "I'm My Own Grandpa?" Kith, or kin?

Bob