but i was thinking more about defending chinese airspace, thats assuming a foregin power attacks chinese cities or armed forces. What about the S-300 or HQ-9, arent those missile securing the chinese airspace even if the plaaf cant take on usaf.
The answer is it depends on the capabilities of the attacker. Let me use the USA as proxy assuming that the "war" will stay conventional.
PLAAF probably has an integrated yet centralized air defence system based on the old Soviet model. Here, radar stations, Sam sites, airfields, and aircraft are under direct control of controllers that provide, early warning, detection, IFF, and intercept data. These assets operate on an overlapping enveloped with multiple assets capable of defending single area. Attempts to decentralized such system (USAF model) is still in the embroyonic phase with Chinese AWACS still in early stages of induction of the PLAAF. (read not yet 100% effective).
To destroy such network, the USA typically uses the "old" standard of stealth and cruise missiles coupled with massive electronic attack. Right now, the US has probable map the whole Chinese territory and knows the location of radar sites, sam sites, airfields, and most importantly communications nodes, relay system, and power grids.
In a time of its choosing, B-2 stealth bomber will sneak into radar gaps to bomb communications nodes and telephone relay system in order to decapitate the centralized air defence system. Simultaneously, TLAM and CALCM strikes will target the power grid and known radar and sam sites and PLAAF airfields. This is then followed by a massive air armada whose mission are to: conduct offensive counter air (shoot down flying aircraft), Sam suppression, and attack of air fields, fuel dumps, etc that are designed to lower the sortie rates of the PLAAF.
The tricky concept to understand is this. Attacks on communications nodes, power grids, and electronic jamming of both wireless communications and radar will hinder the PLAAF general (in charge of airdefence) from doing his job. His attempts to orchestrate a credible defence through coordination and asset movements, etc will be hampered if he can't communicate with his troops and if he can't see what going on in the air. Furthermore, US network centric warfare means that every single aircraft in the US knows what is going on in real time, which precipated a rapid pace of combat. The Chinese general will issue orders based on situations that are already overtaken by events. An order to counter movement A is useless if the enemy is already moving at M.
The result will be individual squadrons or sam site that are fighting independently in an uncoordinated manner. These guys will be fighting blind with little knowledge of what is going on to the airspace next to there own. They will be locally outnumbered and destroyed in detail.
A military force always fights as a team. The US first strike will be to destroy the PLAAF cohesion that it ceases to fight as a team. Each unit fighting individually without support against the US force that still functions as a team will always lose.
