No air defence asset should be viewed as a separate entity facing an onslaught of enemy aircraft formations. In the era of standoff air to ground missile, why we still deploy anti-aircraft guns with a range of few kilometers? Because it is Layered Air Defence System. There are multiple layers of defence with the fighter aircraft forming the outer-most layer and anti-aircraft guns forming the inner-most layer.
Just study the air defence system of an aircraft carrier and you would come to know the concept of layered air defence.
Still if we consider the SPADA or any other SAM system individually, its not necessary that SAM launch unit would be deployed right over the defended target. It can very well be deployed on the expected route of the attack aircrafts but with the SAM radar still near the defended target. If this distance between SAM launcher and SAM radar is upto 10 km, then an attack aircraft can fall easily into the trap because he would locate the SAM unit through the location of its radar but what he would not know is the exact location of SAM launch unit and his "standoff " plans can very well fail. Intelligence is the key to a successful mission.
Thats why major world air forces spend a lot of resources over Electronic Intelligence and do air space voilations when an attack is imminent to "light-up" the hidden SAM radars and thus locate them. Iraq fell prey to these probing flights in the Gulf war and all the SAM locations were revealed. Thats why you need more FIGHTER AIRCRAFT than SAMs.
In the recent past, India voilated Pakistan airspace and this can again happen when war is on the horizon. Just check the airspace voilations done by India before the start of 1965 Pak-India war. These voilations have many technical purposes and its not just limited to checking the reaction time. If a secret SAM radar is activated upon the report of an intruding aircraft, then the location is revealed before the start of the war. Therefore a full-blown use of SAMs is only good for the duration of war. So you need fighter aircrafts to defend the airspace during peace time and just before the war.
For high-altitude air defence system, I would again stress that fighter jets equipped with BVR air to air missiles would do the job better than long-range SAMs. High-altitude recce aircrafts can also be shot down with BVR missiles by fighter jets. So an investement in high-tech jets is better than long-range SAMs.