I don’t agree with your proposals. I’m a reasonably good pilot, at class rank 50. If you’re any good and you’re in a Fighter plane, there’s only two things that can bring you down; other fighter planes and mobile AA. The stationary AAs are easily avoided, and then bombed. Planes that aren’t fighters should also be no match. To have only other vehicles, of the same kind, as your weakness, is not something any other vehicle in the game has. And it’s not following the classic rock-paper-scissors style of gameplay, Battlefield is about. For every threat, there is a counterattack. AA is meant to wreck planes, it’s their counterbalance.
Mobile AA camping in the same spot? That’s good news, because then you know where they are, so you can avoid them. Mobile AA that actually move around is the deadly kind. But they’re camping in a position where friendly assault players can’t take them out? The only place where friendlies can’t take them out is in the enemy home spawn, and if the mobile AA is sitting there, it’s good news, because that means they can’t reach the whole map.
The phrase “playing the objective” means different things, depending on who you ask. To me, it means “do what needs to be done, in order to win”. If the other team is dominating the game from the sky, playing the objective to me means, break that domination. Usually I’d do it in a fighter plane, but if they’re too many to handle, it means spawn in a mobile AA.
A sentence like the one below, therefore makes no sense to me.
They don’t contribute to the team, and at the same time they make sure the enemy can’t use their planes. It’s either one or the other, not both at the same time.
And finally, that no one asked for, a few tips. 
If you are on an AA gun, don’t start shootings at the plane as soon as you see it. Then you’ll only alert the pilot of your presence. Wait until the plane comes so close, that they can’t easily turn around, and avoid you. And if you are a pilot, you should maximize the zoom level of your mini map, and have it to show stationary weapons. If a stationary AA is missing from the map, it means there’s an enemy on it, following my tip, waiting for you to get close enough. Turn around!
Threats and countermeasures, it’s what the Battlefield series is about, according to me.