I think the reason Jack "held back" was because he was tired of seeing his creations bastardized, like Stan did with the Surfer. He was under no obligation to contribute new ideas/create new characters; he was there to illustrate Stan's "stories" - so if there's anyone to be criticize about a perceived lack of originality, I think that should be Stan.

I think Jack understood who owned the characters when he created them for Marvel [and DC in the '70's]. But now those copyrights are about to expire/come up for renewal and he [his estate] MAY have a legitimate claim to a PART of them. If that's what the laws allow, then why not follow up on that? I do not think it's "greedy" of his estate to want a part of the income Jack's creations are generating, if they are entitled to it - and I guess that's for the courts to decide.