As already noted, ET’s success is partially based on the fact that it is free. Without a solid SP portion, the publishers did not want to release it. If you look at it from their perspective, it would have been insulting to their customers to charge $50 for a game that was only half done. The publishers elected to finish the product as a MP-only game and then release it for free online and via demo disks to reduce promotional costs.
I voiced an opinion on why they did it some months ago when the discussion of id/Activision’s motivation came up. It is my opinion that they are attempting to keep the Wolfenstein name in the public eye with an eye to the future. That is, a sequel built on the Doom3 engine. I have no factual basis for this opinion but I believe that it is both feasible and a good idea on their part. In the short term they take a relatively small financial hit, but I think it will pay off in the long run.
As to the opinion that WWII FPS games are on the wane all I can say is that the last time I checked the popularity listing on Gamespy, 4 of the top 5 games were WWII FPS games (ET, BF, CoD and MoHAA). I don’t think the genre has lost ground - if anything, it is gaining.
Dawg