The way you had stated it, and in the context it was found, comes across as if you are speaking to the legality of doing so, rather than the business considerations -- that is to say, it came across as "you can't sell things that include GPL code", rather than "If you depend on GPL code, all your competitors can read your secret sauce".
As to the point of your rebuttal, it probably depends on the design and nature of the game, and on how much of it is defined externally to the source code (that is, in scripts), or in the code itself. I suppose that for very unique features tightly bound into the engine you can argue that giving that away enables exactly the scenario you describe, on the other hand, most commercial games (if that's what we're talking about) aren't terribly unique at all, and do, in fact compete on their assets and design alone. After all, how many games are built on Unreal or Unity? Not to dismiss the significant features that third parties have worked in, or modifications they've performed to the codebase, but this code is an expression of the design, rather than the other way around.
Even with your full source code, they can't clone your game, your assets, your story or characters. For large commercial games asset creation, writing, and design are by far the bulk of the development expenses -- programmer salary is a drop in the bucket, so it's not saving them a great deal of expense. It may reduce their time to market, but its still reactionary and therefore behind the curve you've already set. For a certain kind of game (typically content-light, gameplay-centered experiences) I concede there's a real concern here (e.g. Vlambeer) where a good game with programmer art could be upstaged by a source-clone and better assets, or by launching an identical game with different assets on a different platform (iOS market is rife with such knockoffs of android, PC, and flash games), but those are things that can be mitigated by your own business decisions.
Opinions are entitled to differ, of course, but I just don't see it as that big of a deal in most cases, if for no other reason that the design requirements will be apparent from the finished product regardless. From there is precious little work for a competitor to simply express their own code to achieve the same result. Deliberate exercise of the feature and standard reverse-engineering techniques will reveal necessary detail.