Modern air defences can do their job even without co-operation with air combat assets.
Long-range AD is meant to be a component of air war, not to fight on its own - there are strong synergy effects in the cooperation with fighters and AEW. AD can still do its job on its own, though.
'Doing the job' isn't the same as to defeat enemy air power, that's the key.
AD shall reduce the effectiveness of enemy air power, and that's easily possible if it remains a high threat.
Modern air attack can form strike packages of fighter patrols, bombers, jamming aircraft, AEW&C and anti-AD aircraft (SEAD mission, 'wild weasels') to minimize losses.
That's in itself much inferior in attack capability to a bomber-only force.
It's also quite impossible to keep such an integrated air attack on for 24/7.
This means that no full-time coverage is possible. That's already a huge achievement by the AD as unit movements would be safe during much of the day (the AD needs to be able to tell other forces about the air situation for a good exploitation of this achievement).
Another achievement can be to prevent air attacks at low altitudes. This protects targets very well that cannot be spotted easily, like single vehicles.
Certain air assets are entirely useless in areas with effective AD, even during strike package presence. That would be many drones like Predator/Reaper, transport planes and gunships. Helicopters wouldn't dare to fly high.
AD can also deny the use of certain attack modes and technologies, like neutralization of satellite navigation systems in air attack munitions and thereby worsened accuracy.
AD (in its widest sense) can furthermore reduce the accuracy of attacks with decoying and concealment. The classic decoy technologies flare, smoke and chaff as well as radar jammers can protect high-value point targets against some incoming air attack munitions. Jamming can also prevent the use of datalink.
AD can intercept munitions, motivate the waste of munitions and destroy many drones (like CL289, Sperwer) - this attrition can degrade the enemy's capabilities over time.
Anti-air measures of all troops can reduce air attack efficiency by concealment, camouflage, deception, early warning and dispersion. This works especially well in complex terrain and when there's no ground combat nearby.
Note: It is NOT illegal to hide in cities full of civilians. A correct reading of the laws of war tells that it's only prohibited to use civilians more actively as shields, like herding them to a powerplant or bridge or to move them without any other purpose on trains. It was common and entirely normal to garrison forces in cities, even rather close to the front.
Overall, a proper AD is extremely useful against a superior air power. This is true even without killing any or many planes. The Serbs did quite fine with obsolete material. The Kosovo Air War also showed that against a AD without fighters, the bombers 'always penetrate' - it's easy to bomb and hit large static targets like depots, bridges, factories and power plants. No AD can defend such easy targets satisfactorily.
The best AD equipment to resist a superior air power that has enough SEAD capability is likely
- IR sensors coupled with laser beam rider missiles (range less than 10 km)
- IR sensors/laser range finder fire control coupled with autocannons 30-40mm (range less than 4 km, weapon of choice against drones in the kg range and against nearby stand-off munitions)
- anti-air missiles with active radar seeker, infrared seeker and/or passive radar seeker (no requirement for radar illumination)
- IR, UV and passive radar sensor network with cable communication (independent of civilian communications infrastructure and jammable radio comm)
- multiple AD control nodes
- redundant area coverage (clustering)
- proper camouflage, concealment and deception equipment for point targets (anything from tank up to power plant)
- there is a barrage balloon technology for blocking areas against helicopters (based on kevlar cables that would wrap up on rotors).
- radars for detection and intercept of stand-off munitions (even anti-radar missiles can now be intercepted with proper AD missiles) with command-guided missiles
- radar as lure for enemy SEAD and to create a false situation picture
- finally, we'll see that micro air vehicle UAVs will make shotguns popular. This has been publicly ignored as AD by almost everyone afaik.
Semi-active radar guidance is about the worst of all (and one of the most simple guidance technologies, therefore very widespread) against a SEAD force.
Almost all guidance principles can somehow be countered (only laser beam rider guidance and -for attackers- intertial guidance are not susceptible to practical soft kill), just like fuzes (only hit-to-kill munitions without fuze aren't susceptible to any kind of soft attack on fuzing - even laser prox fuzes can be fooled by nearby decoys).
This applies to the offensive party as well as to the defensive party.