That's wholly unrelated to the amount of threads/messages in a forum, though. And to correlate moddability with storytelling seems bizarre to me. If a game had quality storytelling, why would there be more total conversion mods? Wouldn't a total conversion mod suggest a desire to move away from the game's base "story."
I agree that the number of total conversion mods has decreased since the days of Civ 4 and earlier, but I'd chalk that up to it being harder to do the kind of 3d art that's required for that with the more recent games' graphics, and how much of the game's base code was accessible to modders and when, but not anything to do with the storytelling quality of the game.