Yeah, I think Doom 3 is just id and id supports all their stuff as much as possible. And Doom 3 isn't very old, even if its player base as an MP thing is next to nil.
I wouldn't be surprised if they hire out some company to make a Doom Wars game -- or some Doom MP-based game like Quake Wars, since Id's working on something new entirely...
It's just not old enough for them to abandon yet, since I'm sure they're still selling copies of the game to more casual players who'll buy it because of the name.
That's not surprising.
Doom will sell, based on its name alone. Granted, Doom 3 was good, it just wasn't anything really spectacular. There were some spectacular moments, such as the levels of Hell
, but there wasn't enough of those great moments, to suit me.