People grow up, life happens, interests change, time to move on. You wanna play the same game over and over again? Devs also have lives. It costs a lot to get a game running and attractive (and even then there is cycle of life). It's probably inevitable even in a hypothetical scenario of Westwood never being bought out and being able to continue the C&C series long-term. Eventually people will get bored and just move on. Look at Valve's Half-Life series, for instance. Sure, bigger player base, but this is a fan-based community and not powered by EA Games or Petroglyph Games. I also wager because of certain content it might not be possible to get a publishing on Steam, either. ¯\_(ツ)_/¯
Maybe for some it's just an easy PUG which happens to fall in their theme interests. I am one of those. I have fond memories of C&C Renegade, the late 90s, and the 00s, but those days will never happen again. Times have changed and continue to evolve. The best I can do is dip my feet into something that resembles it slightly. From time to time, some evenings and some days have one of the servers load up with 64 players, and those are the opportunities.
$0.02