It was "conceptually" a great idea in terms of bringing in more people into Natural Selection.
However, Combat was the victim of it's own success for Natural Selection, and thus of course, in my opinion, killed NS. Also, Natural Selection's general learning curve (forget Combat) is a bit steep, so it was the catalyst for the amount of vets and the ratio of combat to classic players being increased to the former.
http://img208.imageshack.us/img208/7089/win2aq3lj6.gif "When God gives you lemons, YOU FIND A NEW GOD!!!"
I agree with foley, CO maps didnt really turn anyone away as far as i can tell because all the vets who wanted to play ns maps just stuck with them and there were definitely enough to keep it alive, if anything CO maps extended NS as a whole.
I believe CO was one of the causes, but the devs messing up NS mode was the main problem. I mean, Nerfing DC's, lowering the average gametime, nerfing fades etc. CO mode also made the community less team oriented and this somewhat showed in NS maps. Recently I've had some really fun NS maps where tactics, skill, teamwork and countering were apparent. It's still pretty rare to have those kinds of games, but the community seems to have patched itself up. There's still tons of dedicated players online, which amazes me.
I was very relieved to hear that the devs are not officially supporting CO mode in NS2, and are only working on making NS mode the best it can be. Very good idea since balancing 2 very different gametypes can be disastrous (as seen in NS1).