WW2 is NEVER going away. As a conflict it is ripe for conversion into any number of simulations or games. It has it all. Global strategic conflict down to individual tactics. It is so well known, so well documented, and has an extensive visual database. Plus it allows for plausible symmetric forces as well as wildly lopsided asymmetric ones up and down the range of focus from rifles to battleships and nukes. It has bad guys EVERYONE can hate and good guys some folks can chose to hate. But it is here to stay. I can't imagine any conflict coming close to rivaling it for collective wargame mindshare.
I agree it would be nice to diversify the engagements but most of these games don't have the fidelity to really make it matter.
I don't think people are saying WWII games are going away, just individual people are tired of the theme. Which is their prerogative.
I'm not really tired of it myself, to the point of avoiding it, but I'm not really seeking it out either -- at this point it has to be attached to a really revolutionary or interesting system to get my interest.