"100% work-for-hire" isn't as specific a term as you might think. It's entirely likely that those creators have contracts guaranteeing them percentages of the work they create if it gets made into films.
Doubtful, as until recently, the contracts that writers and artists signed in that world could be best described as draconian.
Actually I find them to be fairly straightforward. But then I haven't ever written a comic for DC or MARVEL.
A good portion of the concepts and characters that I'm referring to have been adapted and re-adapted multiple times without their creators seeing a dime.
Yes. That's true but a good many of the big comic book characters were created in the 1930s 40s and 50s by and for people who had no idea what would come of their work. They signed work-for-hire deals accordingly. I don't have any sympathy for those who "got the shaft" because, at the very same time, one Will Eisner was protecting himself and his estate from just such eventualities. You snooze, you lose.
I'm willing to bet, sometime after SUPERMAN (I), creators and especially creators who were also editors (O'Neill), began working percents of percents into their contracts when it came to "derivative" or "ancillary" works. The formation of IMAGE comics was partly due to the intractability of Marvel on the point of paying creators what they felt they were owed on a work-for-hire creation.
The point is, one size certainly does not fit all.
Now, though, a lot of creators got checks, and Jerry Robinson (creator of the Joker) has a position as an "Executive Consultant" at DC, despite the fact he hasn't contributed much of anything to the comics field in thirty years. It's not because Warner's fears legal action, it's that they don't want another debacle like the Siegel/Schuster disaster. It makes them look like the good guys.
Up to a point. In reality it boils to down to bean counting- cost/benefit ratios. It's never about being sweet or just. Never.
The point is all of us sign contracts that we've presumably read beforehand. Nobody's getting shafted by an agreement they knowingly signed off on. It's part of the deal. It IS the deal.
And, of course, there's always just making up your own univeres and jumping into the big pool as more than a few Trek (and other media) writers have done. In that venue all bets are off. you own everything and can peel off the bits you like for licensing or tattoos or whatever you like.
I like writing Trek becaue I've been Trek-obsessed one way or another since I was a kid. So, if some movie decides to focus on the Seleneans and I don't get "my fair share" of the take from said blockbuster, well, I know I invented them and added to the tapestry. Nobody's taking that from me. It's not always exclusively about money.
But, if i can get into position to write canon material, so much the better.