I believe that when, and if, a modern RPG case is ever litigated over copyright issues, that case will write all-new law, creating a whole new category of copyright protections. It will have the same kind of impact that the software copyright litigation did. The courts have never seen anything like a multi-book TRPG, and "simple" rules like "games can't be copyright" will be tossed in favor of a much more nuanced examination.
I even think it is possible that lower level courts would look at a TRPG and essentially say "that's a novel, not a game", and proceed from that basis (the result would be sweeping and massive increases in the protectability of RPG content, but then you'd have to untangle 30 years of publishing history where games are clearly derivative of one another without licenses or intra-publisher agreements, and that could get real messy, real quick.)
The arrow of law points in favor of the publishers on these issues. As intellectual property has become more and more important to the global economy, courts have been increasingly likely to extend broad & strong protections to IP ownership. The odds that RPG content will be found to be protectable increase steadily and there is no reason to suspect a reversal in that trend is likely.