I have mixed opinions on this. I hate corporate culture and the pure greed involved, however I do not always think big business is wrong. What's more, especially when dealing with old work, sometimes decades old, I do not think it's right to retroactively apply modern standards and viewpoints to something that already transpired. Kirby was writing in the 1960s and his situation and working environment wasn't unusual, it was the success his work lead to. Coming back with developments and attitudes decades later, based on a huge windfall created by some of his creations, strikes me as being wrong. One of the big principles of the US legal system is not to retroactively apply laws, and while it does happen (and is even legal to do in some cases) I do not generally support it, especially when dealing with cases of estates and long-standing copyrights and IPs.
The way I see it is this, if one argues that every writer or creator who sells a story or piece of art is entitled to a share of the profits should that work ever wind up making a ton of money, it would become nearly impossible to hire or print anything. Every single time you hired a writer or artist, effectively speculating on their work, you'd be taking a risk. The point of hiring something to make something for you, is that it's done FOR YOU, not something they are lending you. What's more even today when you submit to magazines and the like it's made very clear that your selling the publisher your work, not entering into a partnership with them.
Kirby was pretty much involved in a subset of the magazine trade for all intents and purposes, something people generally make pennies at. What's more, comics at the time were not that big as a competitive market. Simply being able to make money, reliably, off of what he was doing was a big deal, and truthfully if his stuff hadn't succeeded to an unprecedented degree, he probably would have been happy.
From the perspective of a publisher, these guys tend to pay out tons of money for garbage that goes nowhere, hoping that maybe, just maybe, for all of their overall investments something will "hit" and turn out like Kirby's work did. A lot of publishers go out of business engaged in this kind of speculation, and as such it's not surprising that when one has their investments pay off, they are reluctant to give away a huge amount of the money involved to an initial creator. After all the creator was profiting when he sold you his work, while you were the one gambling and taking the biggest overall risks with the publication.
Now ideally I do think they should have treated Kirby better. I don't think his demands (which amount to a direct cut of the profits and/or control of the work he sold) were reasonable, just as I think Marvel's counter-contract was kind of insulting. That said I think they could have worked harder to come to an agreement, it seems like it was a clash of egos, and the bottom line was Kirby preferred to storm off than simply take a payday and a job and end the conflict there without creative control.
The Kirby Kids do strike me as being greedy, because their push is largely fueled by the massive success of the current movies. Back when things were a lot less profitable they didn't make a push when it would have been more viable, because they want the money, not the rights per se, it's about money in the bank, not the principle of the thing. The timing here pretty much says it all. There have been decades where this stand could have been taken, but it wouldn't have had tens of millions of dollars as a potential payday. Whether there is a legitimate point buried in here or not, the point is there is very little nice people can say about the kids and their very transparent motivations.
As a fan, I prefer the right stay under one banner, and have a few different management voices as possible. It's already bad enough that we have three groups (Disney, Sony, and Fox) all sharing Marvel rights and apparently bickering with each other, without the Kirby kids coming in and becoming a fourth force in all of this. Truthfully I wouldn't be surprised if in the end it formally ripped The Marvel Universe apart, and really the collective IP is stronger than the sum of it's parts. Given to the kids, I think they would just grab the profits, sit on the rights with high hopes, and we'd really never see much done with them that approaches the glory of the unified Marvel Universe. This is not to say that I love Disney or any of the big companies involved, or anything like that.
Let's be frank here, up until fairly recently, writers producing incredible works and dying penniless was a painful stereotype for a reason. Kirby was a creator at a bad time, and I do feel sorry for him, but that doesn't mean I want to give his kids Scrooge Mcduck's money bin and tear apart The Marvel Universe. He pretty much dealt with publishers like nearly everyone else, and the big thing is that he and his family are bitter because his stuff sold on a mega-level where most people writing never did and were happy just to get paid and see themselves in print.
I'll also say that Kirby is a bit of a character himself, as the article pointed out he was apparently bitter over Stan Lee being more of a "face" than he was and not getting much credit. That's kind of silly when you consider that most people who know comics know who Jack "King" Kirby is, right up there with Stan Lee. While he might not have gotten the money, he DID have the fame, and really I don't think he did a lot to leverage it, which is kind of on him. He seemed to become increasingly bitter, as opposed to adapting. If the guy had rallied, taken some business classes to learn more about that side of the business, and then started leveraging his own fame better, things might be a lot different. Half his problem seems to be that he was sulking, if he had produced enough work of similar value, he could have then negotiated based on "well if you want this stuff to make money on, then I want a cut from my older stuff".