TL;DR: Air has changed a lot recently so changing AA is probably not a good idea right now. Flak and Hacksaw do have (narrow) niches so the only real reason for a change would be to foster gameplay where AA can be sniped by land forces to facilitate bombing runs and more cooperation.We've tried many times to dislodge Razor is the go-to AA turret, since we want singular points of failure for AA that land players can snipe out to facilitate attacks from their air allies. The Razor was meant to serve more as a dependable backup in games where there is artillery flying everywhere and you just want something that can at least fight back. However, it's usually as much as you need and when you want to add more AA power, you just add in more razors or jump straight to Chainsaws.
I think this mostly comes down to coverage-for-cost. Two razors cover a lot more area than one flak does, and the DPS and AoE (Which you rarely need) of flak is just not enough utility compared to the utility and safety that the HP of a razor offers. This is why people jump to chainsaw, it offers superior coverage. However, upping the range of Flak isn't a total solution: The Razor is just at the right cost point to be a good first response to enemy air (About the same cost as a plane/gunship).
Hacksaw, on the other hand, is meant to be a spot-defense. It's meant to defend single high value targets, for when you absolutely need to kill an incoming bomber. Google nerfed it because he felt it muscled out bombers, but Ravens were too strong, and now that Ravens are nerfed I think we have a much healthier air game, a higher choice in second factory, and more dynamic game that doesn't lock you down under your static AA cover, and less oppression of high-cost low-HP units like Mace, Penetrator or Pillager: Not to mention comsniping. I think this is much better than having that strong counter mechanic where Ravens are mandatory and hacksaws are also mandatory.
But with the weaker Raven, Hacksaw is better against it. If it's not worth building because there are not enough bombers in the metagame, that's probably fine: But it should probably still be alright for when you absolutely must kill that Thunderbird, Licho or Raven. We don't need to discourage bombers any more than we do.
Maybe if
GoogleFrog still feels the same way about that design goal we could look at Flak vs Razor to facilitate a gameplay where land forces or artillery can surgically take out AA: Maybe to the point of just swapping their cost and weight. But air is in a very new, very different place right now so there is not that much need to shake it up: Flak does still offer the utility of AoE, which is sometimes a thing.
Gotta say I don't mind Pxtl's suggestion though, killing planes before they drop a bomb, or after they do to assure attrition, is kind of the only interaction you have with them. Would be nice to have a way to deny air in an area without the plane player suddenly losing his whole investment.