16 years ago, when I first discovered Doomworld, I was surprised THEN that the community was as strong as it was. It's still going, I mean I popped into #zdoom, and was surprised that yep, the same ol cast of characters are still there. I honestly believe, if things less mod friendly like NES gaming and the likes are still as popular and going strong, Doom is NOT going anywhere. Actually, I see Doom being referenced in the past 10 or so years in mainstream culture, far more than I ever did back in the day, and even then I saw it a lot.
The game is almost as immortal as Hellenistic philosophy and literature. Actually at this point, it may as well be a memetic virus, being ported to everything and anything that is capable of computing on any level.