One of the most amusing things you see moderating Forums around is people desperately trying to defend a status quo that favors them, even when clearly in the wrong. Just a random thought...
Anyway, it's true that there's nothing wrong with the AFK system - if it's taken on its own, outside any context. When you add in the premade factor that isn't true anymore, as those more than obviously abuse it to get rid of people in their side. Going with the premise that the AFK system is fine, it's a smooth segue to conclude that premades aren't. So, if reporting individually is inviable and the AKF system should remain the same, what would be left is dealing with premades instead, yes? Which, full disclosure, I'd personally be completely fine with.
As the chances that premades will be limited or made illegal might not be that big, unfortunately, it falls on one of the two other factors to be tweaked - that's plain being logical and honest, staying away from disingenuousity. Ideally, I'd say the best case scenario would be for it to be in the reporting. With targeted reports being unfeasible due to the lack of proof to finger someone specific, a premade-wide report would be great - they are in it together, they share the same punishment for abusing the feature. Any claims of "unfairness" are moot, since they are after all organized, often for a long time now, and everyone in one should be aware of their methods, likely adding their own vote for the AFK kick.
Barring that, it's the AFK system that requires adjustments, by elimination if nothing else, yes.