The title feels a bit off. I doubt very many of the subreddits have actually "relocated". As the site itself states, these are just lists of alternatives that provide topics that match existing subreddits.

Most of the subreddits mentioned are still going to be active (especially in comparison to their alternatives) for the foreseeable future.

I think you've summarized the biggest problem with Halo currently. Even if you enjoy the game, its one if those games you play for a while, maybe beat the campaign and then move into other games.

It just doesn't feel like a flagship title anymore. And if it wasn't for the Halo name it would be even more forgettable then it is now.