The O'Neill cylinder have a few problems in its design, the 1st is that a lot of important stuff is concentrated at the tip of the station and not nestled under several meters of armor or non-critical sections. Any attacking fleet can see the Radiators and the satellite dish and figure that, that looked important and concentrate their fire on that. Yes there are shields but better to have redundancies than rely on 1 system. 2nd problem, the huge af windows are unnecessary, an artificial holographic sky is good enough. 3rd, the food modules being outside the station, I remember reading that GDI used anti-grav from the Sandstorm propulsion and in the C&C3 Philadelphia cinematic you can see people walking inside the station through the windows so artificial gravity is something GDI can make without needing to use centripetal force. I think the TibWar2 era space station already have artificial grav too.
The idea of having space colonies is fine, but there is the opportunity cost of building a space colony, when those resources could be used to colonizing a planet. Colonizing a planet is much easier and cheaper than creating a space colony from scratch along with not needing to worry about a population limit due to how small a Space Station is in comparison to a planet. GDI will have Space Stations but they are mostly ports, refueling stations, and GDI Navy Bases, not for civilian habitation. Civilian Space Stations do exist but they need to have a permit to build a station there along with having it inspected by GDI Engineers so that it is up to space station building code. Thus only the extremely wealthy and large corporations can afford their own Space Stations.
