You guys are forgetting a few things as far as I can tell. You're confusing how many people would buy the game on DVD with how many won't buy it on CD if DVD isn't available. That's the two costs you have to compare. If the lost sales from a CD version is less than the estimated production costs of having two seperate manufacturing processes, then financially speaking just going CD is the route to go.
Of course it would be nicer on DVD, and of course they are going to make a boat load of money, but lets get something clear. We shouldn't be criticising a company for doing what the majority of companys are, we should be applauding the companys that supply games on DVD.
The analogy about a movie on 6 cds is a moot point since you install games and then only have to give the game a single disk subsequently. a film you'd have to change disc 5 times every time you watched the film.
If you're going to deny yourself a potentially great game over having to spend 20 minutes longer installing it, that's a pretty bizarre thing to do.
And as for the multiplayer, as others have pointed out the 4 player limit isn't hard coded into the engine, but it's probably the limit at which the game runs acceptably with everything going on in multiplayer. I've heard some descriptions of maps that have huge sections of the map moving around for example. If we have much more complicated dynamic maps and physics that's a lot more information that needs to be sent back and forth across the network and more load on the server. So it's probably more to do with hardware than engine. No doubt with simplified maps, and models the current engine would be able to cope with more. I'm interested to see how it plays but... and a big but...
Contrary to popular belief iD do not have to supply any kind of multiplayer at all. Infact when they started out on Doom III they weren't promising any. They said, that as far as they were concerned they'd made the best mp game with Quake III and so for their next game they were going to focus entirely on SP. Heck, iD didn't even make the multiplayer component.
As for talk of modding... here's how I actually see things. Do people seriously think that Half Life would have anything like as big a modding community as it currently does if not for the fact that it was the best single player game in a long time. two get a mod community you need two things. a big install base and an easy to work with engine. i'm betting doom III will have both of these things, albeit the work involved in creating a level or model of the standards the game is presenting looks much increased.
and finally... what's wrong with AvP wrapped up in an utterly beautiful engine anyway? AvP is one of the best PC games I've ever played.