Half-Life's versatility made it so popular. The plethora of Half-Life mods has shown Half-Life's aging engine does not deter people from playing it. The most versatile engine (currently), has to be the UT2K3 engine. Its pretty, has all the animation and physics control anyone needs and is already established out there in the gaming world.
Granted, Doom3 isn't out, but UT2K3 is out, has other developers making a games built off of it and already has a community making mods for the system. Sure, getting a CStrike-like mod is going to take some time, but it is very possible in UT2K3. My only beef with that game is the performance hit seems unreasonable. When I play America's Army, UT2K3 or even the Raven Shield MP demo, my system just severly slows to a CRAWL! I have a "decent box" (Athlon XP, Radeon 8500 128 meg) and anything built off the UT2K3 turns my machine into molasses in January with only medium settings!
Sure, a handful of people say "upgrade", but upgrading isn't always an option for everyone! I no longer am willing to upgrade my computer bi annually, or annually, just for the latest tech. Ut2k3 will be pretty nice, on my "next" computer, but as it sits now, I am waiting for a cool mod for a reinstall.
vash