1) It makes perfect sense. Groups like ISIS and other terrorist groups only operate in weak nation states or semi-failed nation states or regions. Look at the groups that cause havoc from Mali to the tribal areas of Pakistan. The same problems can be found in all of those places.
This is not the case in KSA ANYWHERE.
2) So you really believe that ISIS will suddenly move all their focus away from Eastern Syria, Al-Anbar and the Al-Jazira region in Iraq just to attempt to roll into Northern KSA? The same Northern KSA that is one of the most heavily militarized zones of the ME and which has amble military bases, airbases next by? Let's not even talk about geography. Despite numerous valleys and highlands here and there in the Northern regions/provinces the territory is mostly steppe/desert or just flat grasslands. It's ideal territory for an air force to do what it does best or for an professional army to do what they do best.
3) Yet KSA is one of the few if not only major countries that have managed to clamp down more or less fully on an local insurgency like KSA did in the 2000's. Yet ISIS has been causing riots in Iraq and Syria for at least 2 years now and we have only seen 1 attack in KSA since then. On a border post near Yemen. This speaks volumes. Besides ISIS has minimal support in KSA. People value stability and prosperity over what has been seen during the Arab Spring especially in nearby Syria. Nobody is interested to repeat such a stunt and there are no real initiatives that can cause such an uprising as KSA is in a stronger position than ever as is the whole GCC. You are underestimating AQAP. Before the rise of ISIS they were the most powerful terrorist group in the ME if not the world.
Anyway I have to go and we are off-topic.
1). Many of the IS recruits come from STABLE states. Essentially normal folks who get hooked onto the idea of a united Islamic state by force, nothing prevents that from happening in Saudi Arabia.
2). They wont move their focus, this is a gradual move and their ultimate goal. Your 20% increase will not land 500 tanks tomorrow morning. It is also a gradual increase. You clearly need to look at cases of guerilla warfare in Algeria, Libya and most recently Iraq as well. Despite ample air power in these areas, it took boots on the ground and a lot of casualties to accomplish some sort of stalemate against what were essentially 1/10th of IS forces.
3) IS is not at all targeting stability and prosperity, it is targeting a a general frustration with events. Which is visible in the trend of recruits coming in. AQAP was laughable at best with 500 terrorists looking for terror activities. Here you are talking about some 10000+ fighters capable of waging both guerilla and conventional warfare..and this is the current state. What of two years, five years later?
Guerilla tactics in an empty desert!!! Are you not aware of KSA, Iraq geography?
Conventional warfare against the most powerful air force in the region?
Not sure if your being serious.
Perhaps you need to read some history. And learn about the limitations of air power.. but that requires some serious brain power.