Different weapons for different tasks.

I think that we may be violently agreeing. What I had in my original comment:

They may both have a role.

If you know that a given point is at risk of attack, using a static defense like AA guns is practical. Say you have some sort of specific, high-value target that you can put AA guns around. That may be a very sensible thing to do.

But the problem, if you intend to rely only on those, is that there is then a concentration of force issue. The attacker can choose which point to attack; they get the initiative.