You don't need to have aircrafts patrol the airspace all the time to know hostile or unknown aircrafts are coming. If you have radar networks set up properly, you can have full coverage of what's coming at you at ALL TIME. Namely, the US and Australia both has active OTH systems (Over-The-Horizon radar) that can detect any incoming aircrafts over 3000~5000 km away. The Australian's JORN is so accurate it can actually detect fluctuation of the sea wave, or Chinese missile launch 5500 km away. With OTH systems in place, any aircraft coming your way you can quickly vector in interceptors before they are even coming close to your border.
The thing is, I thought China has quite a few OTH systems... supposedly. I guess they are not pointed the right way
Over the horizon radar is considered useful for long range use against very long range contacts, yes, however I do not believe that any OTH has been developed in a way which can be operable with the clutter discrimination against a low altitude target that a true look down radar from AEW&C can do.
There have been some quite amazing claims of Australian OTH radar sets being able to detect planes landing in east timor 2600km away, however they're still obviously operating a fleet of new AEW&C.
So I think in a tactical and operational, real time sense, AEW&C is still the dominant platform of choice.
I do think the PLAAF can invest in large long endurance blimps in future with radar sets. Maintaining a high altitude with a large look down AESA, they could provide the kind of long endurance large area coverage the PLA needs... but this is all in the future.
Fact is at present the PLA simply doesn't have the capability to monitor its borders to the detail that it would like. Namely, against low flying targets against clutter and other physical impediments such as hills.