WW2 is not a compelling enough "theme" to install a 4gb mod for most people. It's done in too many video games and unless the game play is totally unique or it offers some deep persistance system or hardcore teamplay, people will not be able to see pass the expiring date on the actual game they launch.
Well maybe you (as the devs, I mean) should start working on those "totally unique gameplay or deep persistance system or hardcore teamplay" then...
I'm not saying you're doing a bad job, in fact I admire all the hard work that has been put in this mod, but it seems to me it still lacks some "new features"... Sure it has awesome graphics, excellent historic accuracy but the gameplay still plays out a little too much like BF2...(as in you spawn, run or drive around for a bit, cap some flags and eventually win by spawnkilling the enemy at their main/1st flag). It lack something new and different... Of course push mode has offered some concentrated action and ceased the back capping but I still feel it's a bit lacking.
Maybe it's not the all devs' fault, maybe it's all in the community (maybe that's why tournaments are so popular) but I find it hard to believe that devs don't have some nice ideas to "spice up" the gameplay.
And about your dying out mod theory, we got to take a look at the big picture, sure gamers are moving to newer games, but some of them also come back disappointed... Also new gamers are made every day... Hell I'm a newcomer, I bought my new rig mainly to play FH, not for some new generation games as they all started to feel exactly the same for me... Specially with the focus on the modern part of history.