I think Rage is just not a multi player oriented game though, is it? Doom 3 sure wasn't. Had like, 8 person deathmatch is all.
I think it might even be 4 actually. I know the XBox version could only serve 4 players. The thing with Doom however, was that the game was more oriented for professional gamers and included shader hit detection instead of hitboxes. Hit detection based off of shaders used up a lot more resources than your average hitboxes and professional gamers don't play more than 1v1 anyways.
Servers are definitely not a remnant of the old days. They are a remnant of the PC gaming community, which tends to be older and more traditional in terms of preference. If Carmack wants to appeal to the new market, then he can do it. However, I will not buy a game that isn't aimed at people like me and doesn't have people like me in mind when there are tons of other games that do meet this criteria.
