Not sure what the Avengers-film-$$-Kirby hoopla is all about.
Agreed that he got screwed way back when and decency should include his name in the credits as creator (which it was), but as to the money from the film, neither he nor his estate had anything to do with assuming the financial risk, hiring people, creative decisions, marketing decisions, logistics, writing, acting, or otherwise producing the film, and they didn't have a royalties contract for film reproductions as far as anyone can tell, so there really doesn't seem to be anything to argue about.
Stan Lee had managed to arrange the last two, so he gets paid. Good for him, and just too bad for Kirby's estate.
Do people send the Anchor Bar a royalties check every time they eat Buffalo Wings?