Ask the pilot of said multi-million dollar plane why they tend to stay above 10,000 ft, when there's a lot a AA around. You may not hit much with cannons these days, but it still make good harassing fire. And it can be use against ground targets.

Dancing Johnny has a couple of points well put. My position is: In air defence, don't ever look at one weapon system at the time, what counts is the combination and coordinated use. AAA are very effective against helicopters and "lift" the enemy into higher strata, where the fighters and SAM can get them. That is very important. If you have ever seen a Tornado on the deck going at a respectable speed, you know that the first thing you will see are two little red dots of a Tornado getting the hell out of there with a speed that makes it difficult for missiles and shells catching up before the plane is out of range. The effectiveness of AAA depends on an Observer Corps giving 2-3 minutes of warning so the guns are manned, keyed up with the barrels in the right direction with minimum deflection that is - according to reputable pilots - in itself scary, when the world becomes even more interesting, as a Stinger-type unit decides to join the action. Both will probably miss, but you might have to go higher where SAM and fighter await you - and they were the reason you decided to avoid medium level in the first place.