Can you provide an example of that kind of harm? The link does not seem intuitively apparent between "white people generalize, bowlderize and misunderstand some cultural stuff, but manage to make it highly marketable for aforementioned white people" and the marginalized group loses money they would otherwise have.
Most arguments and examples I have seen for the economic angle are focused on the unfairness that the marginalized group does not profit as the privileged one does, not that the marginalized group loses money.
Not sure why this is complicated, but I okay I guess some people have difficulty with this. Obviously yes, there's the unfairness you describe.
But culture is like an IP. Particularly cultures that are somewhat obscure and have some broadly interesting myth/stories/ideas. That IP has value, and that IP can be damaged.
If I write, say, Choctaw Adventures, or whatever (I swear to god if anyone tries to argue specifics on this entirely theoretical example I WILL block them and never think about them again), and I use Choctaw myth and so on, may misunderstood in some kinda crummy ways, but let's focus on the economic, because it's what you want to know about. I don't share the profit with anyone from the community or whatever, so you've already identified a harm in that they don't profit, but I do.
But there's more than that - if someone from that community had wanted to do a Choctaw sourcebook, well, too late, there's already one written a white guy. Any one they put out is going to struggle to get out of the shadow of that. Because it's not the first, it's probably going to have to be twice as good to attract half the notice. And even if it's more authentic, or even just cooler, because there's going to be a ton of overlap, some people are just not going to be interested.
And it could be worse - the person who wrote Choctaw Adventures could have really mucked it up, and make the IP toxic or crummy-seeming in some way, like maybe he told the myths very poor, or made them seem boring or dumb, so he's damages the IP as well, so then if an actual member of the marginalized group wanted to write his Choctaw sourcebook, it's even harder for them to succeed, and it may be that the idiot who took your myths has managed to convince a bunch of people that they're dumb and boring (or even just that they know them already when they don't), which might even damage projects in other media.
Then on another level, there's the issue of "official" books. Say WotC put out Choctaw Adventures (remember this is theoretical people, don't make me Block you!), then that's going to massively boost the sales of that book. But say you come along and say to WotC, guys, this is the bad kind of cultural appropriation, and misrepresents our myths in a really crummy way, let us do a better version, chances are, they won't want to, because they already did a Choctaw Adventures, and it sucked (or succeeded, it doesn't actually matter), but they're not going to put out another book on the same subject in the same edition, so your community double misses out.
So do you see how that's harmful to artists and creators from that community on a level beyond the unfairness you already identified?
Now, note, this isn't always bad, particularly if you work with the community in question, and particularly if you just take one story or one myth, and you do a really good job - because then you can actually increase interest in that marginalized culture. But historically that's been rare, and typically requires a fair bit of luck or exceptional judgement.
