Fighters are the best counter against other planes, yes. But I’m not sure you realise, that’s not a good thing, it’s a sign that asymmetric balance of rock-paper-scissors is not working as intended.
There are huge BR gaps between AAs. Most are over-BR’d and subject to nerfs (no stealth belts, no bespoke reticle), and at a range disadvantage against the planes they are supposed to counter.
In order to be effective, they need to leave spawn: good luck doing that if your team is losing. To reload, they need to drive to a cap, like tanks, even though it is manifestly much more dangerous to them. There is the ongoing problem of how to make them effective against planes without turning them all into tank destroyers.
Finally there is the issue that if you spawn SPAA first, you will likely not contribute to the first engagement (and you’ll get bored). But if it’s your second spawn, a plane has already killed you. And even then, you have to choose between tank (help your team with the objectives or try to turn the tide if you’re losing) and SPAA, and if you choose wrong, it can cost you the game.
Sorry, but in terms of game design, this is not a counter. It’s a band aid. A Dicker Max’s gun is a counter to the Churchill’s armour which is a counter to the mobility of a medium which is a counter to the unarmoured Dicker Max, that is asymmetric balance. SPAA is not performing like that right now.
By the way, you can get empirical proof of this. An SPAA costs between 1/5th and 1/10th of air to spawn. Why do you think that is? What imbalance is that cost difference trying to compensate for? Considering that Gaijin balances by performance and efficiency, if a unit costs 1/10th to spawn compared to what it should “counter”, what does that say about their respective efficiency?
Think it through, and draw your conclusions.