Although the GPL is a chaining license, that license likely only applies to derivatives. While, AFAIK, there's never been a copyright case (and GPL is a copyright matter) in which someone has ever tried to successfully defend chaining licenses, they'd likely succeed, just because the license is explicit and violating the terms of the license means creating an unprotected work which is a perfect case for infringement.
However, the important thing to note is that OpenMW's software is the only thing protected by this copyright law. As we see with OpenMW itself (which, by the way, doesn't even need Bethesda's permission, because it's clean room reverse engineering which is usually legal under copyright law, particularly in the US which is where we'd be likely to see the matter go to court), the data files used in OpenMW are not protected.
So, in short, being under GPL just means that future improvements to the OpenMW have to be made available for merging back into the project and other projects. However, you'd be hard pressed to find a court that will find you infringing for providing OpenMW and game assets for it, and then protecting those game assets under a different license.
For comparison, while Morrowind and its data files would likely be protected as one program, transferring either the executable or the data files on their own is still infringement, as the data files consist largely of protected works (audio, visual, et cetera). The game mechanics are unprotected, which is why OpenMW can create a platform to allow people to load up and play Morrowind, so long as they don't distribute anything Bethesda made. As a side-note, mechanics in general are unprotected, as we see with ReactOS/Windows NT.
So, in short, there's no reason why the GPL should prevent anyone from building upon OpenMW's engine. Sure your work on the engine will be under the GPL, but you can still sell stuff under the GPL and protect your content without any issue. Now, if the OpenMW team decided to write something into the license for OpenCS that restricted content created with it to being released under a certain license (as content created with the official construction sets probably is for noncommercial use), you'd be fine.
There was only one case where compatible game content was ever actually barred from being sold/resulted in damages being granted (which means it failed to qualify as an original, and separately protected, creation), and the argument relied on the final product (an unofficial Duke Nukem expansion) relying on the original Duke Nukem content rather than the fact that it was interoperable with the engine running it, and quite frankly it's an opinion that is likely to be reversed by a more tech-savvy court.
Or, perhaps most importantly, from the GPL v3 itself:
EDIT: Of course, there are some caveats to this:Inclusion of a covered work in an aggregate does not cause this License to apply to the other parts of the aggregate.
1. You must make it clear that the GPL-licensed content is under the GPL. That would be the OpenMW software. GPL requires you to include the license, and I think the source, though I don't remember if you have to distribute the source with it (i.e. pack it into a game folder on disc/digital distribution) or simply make it available.
2. You must make it clear that the stuff you want to traditionally copyright protect is under protection. Be sure to specify whether you reserve all rights, or place it under something like Creative Commons, and if you choose to allow further modification be sure to state what assets can be redistributed for the sake of modification (see Bohemia's stuff for licensing Arma content to be used in other Arma titles).
3. Include explanations for your changes to the GPL-licensed content. This means you can't sneak in "well, here's some netcode, and hopefully nobody finds it and steals it for their own OpenMW interpretation" and still be okay.
However, another thing to note is that some open licensed stuff still sells. Look at Eclipse Phase in the tabletop gaming market; it's sold a ton of games and become one of the more famous tabletop role-playing games, and you can download it for free and toss it to your friends. It's the "positive sharing" side of open licenses; if they like it they'll pay you.