That's an excellent point, and it brings into question what qualifies as "AA". Quad-50's and Stinger missiles would be largely ineffective against MBTs. On the other hand, the German 88mm gun was very effective when employed in an antitank role.
The difference, of course, is the means and method of delivery; the Stinger is far more effective as an AA weapon because of its specialized guidance systems and payload, but that specialization means it's largely useless in any other context. The Quad-50s (or quad-20mm or whatever) is probably the least effective AA system, but can be devastating against soft-skinned and infantry targets.
I think far-future AA systems will either be of the guided-missile variety or energy beams (laser, graser, microwave, particle, etc.) which are instantaneously effective without the complicated ballistics (and Kentucky windage) required of projectile weapons trying to engage a fast-moving aircraft. A third option would be some kind of thermobaric explosive charge or whiz-bang weather machine that creates such a disruption of the air in front of an aircraft that it loses all airworthiness and has a good chance of crashing. Oh, and a fourth option would be an EMP weapon that incapacitates the aircraft's systems, causing it to fall out of the sky.
Of those, the missiles and air-disruption weapons would be of minimal usefulness against ground targets. The energy beam and EMP weapon, though, could do some significant damage. Even infantry would be vulnerable to EMP attack if they were kitted out with electronic sights, exoskeletons, onboard computers, portable radios, etc.
So, without more detail on what each Iron Cow AA unit is carrying for specific armament, it's hard to determine what its effectiveness might be against anything other than aircraft. Makes for a neat intellectual exercise, though.
I do agree that any point system would need to be playtested using some standardized "average" unit. Some of the assumptions in the point system I came up with rest on the concept of an "average" unit. The Speed Multiplier, for instance; I took an average of the speeds of all the units shown on the IC page and it came out to 248-point-something, so I figured that I could use 250 as the base speed for an average unit. Dividing the Speed by 250 then gave me a measure of the comparative amount of power needed to push the unit along. Multiplying that by the Defence Factor was then used to estimate how large the powerplant would need to be to push a unit that solid that fast.
Leo: (July 23—Aug. 22): The wheelchair and the indignity will be bad enough, but the worst part is going to be explaining to your wife exactly what you said to the genie to make him take off your legs like that.