Id Software has been struggling with the return of DOOM for some time. I'm not surprised that the game turned out the way it did. You can see the focusses of the development and where they had to sacrifice to get the title out. The number of gamers who are in the main buying market who remember the original DOOM are growing less, as less computers can run it and it is not on the shelf anymore. They needed to get this out before people lost sight of the origins (which can be fatal to a sequal game).
There's actually been a compiled history of artwork, code, and storyboarding for DOOM3 that extends back a good 5 years. Given that the original DOOM had its luster days at the beginning of the 90's, you can see how long its been. If your target market is teenagers, you can see how there can be so few who have actually played DOOM. Most were still toddlers when the craze was ebbing.
Still, now that the craze is back on (for now) I'm sure we can expect to see extended development of this game in the future. There will definately be work on an expansion, and we all know the gold/platinum/super editions are coming.
For now, the real question becomes how good is the game itself, in regards to coding, AI, modification, etc. A video game today cannot survive if there is not a sufficient fan following and hands-on interest after release. One reason Half-Life has survived so long is because of its incredible ability to be customized and formated to fit near-limitless ideas.