The issue with this is how high of a % of wizards would you need to have to cast the reduction spell?  You need a ton of wizards often times to get the estorms to pass once people are up and going and can defend themselves.  If you don't make it a crazy amount required to cast the reduction spell then it makes being a pax or using the ops not worth the time.  As well as if you make the amount needed to pass the reduction spell does it then make it not worth bothering to try and reduce it because you are losing a ton of upkeep income by having to have that many wizards to cast it?

Modification to the idea!

Banker A wants to protect its pop from Pax A.  Banker A asks Pax B, from its own fam, to protect it.  Pax B casts "Assist Ally Defense" or whatever the hell we want to name the op.  The op doesn't outright prevent spells.  Rather, it records the amount of wizzies Pax B used in the op, and uses that as the banker's defensive number for the next wizzie op against it within the next 8 (or 5, or however many you want) ticks.

1: This solves the problem of how many wizzies are necessary to complete the op, because it makes the op's efficiency still require proportionality to the attacker's wizzies.
2: The op involves multilayered strategy.  A defender has to decide whether they want to focus on protecting their bankers or opping the opposing bankers.  Sure, the opper can protect their ally pretty easily.  But then the other side's bankers get a free ride!
3: Additionally, this creates an interesting dynamic when a defending pax has to manage multiple bankers.  If I'm protecting two pop bankers, should I protect them both evenly or favor the bigger one?  After all, protecting each with 3 ops will stop someone who decides to target one banker, then switch to the other once they realize the first is protected.  On the other hand, perhaps that attacking pax would just plow 7 ops through your defensive ops anyway... 3 fail due to your protection, but the rest break through.

The problem, though, is that there's no reciprocity.  Someone building a massive fighter fleet can be countered by building more fighters or stationing fighters.  Massive ground?  Either out-ground them or have fun trapping the target with fighter fleets.

Agents and wizzies are the exception.  If your bankers don't have sufficient defenses against them, you lose.  There's no alternative method of stopping wizards.

Additionally, because this threat exists, the actual act of using wizards is damaging enough in itself.  If I build 10 million wizards, a fam that suspects I hate them has to waste resources to build 5 million soldiers on all their relevant bankers.  That's a resource hit.  Then they have to pay upkeep.  That's a hit.  Meanwhile, I'm screwing around with other fams or doing random PIs, knowing full well that bankers in the fam I hate are blowing a solid amount of income purely for deterrence purposes... and if they let their guard down?  I reign hell on them, and there's not a damn thing they can do about it.

So no... I don't buy that it's a "a big risk" that's justified.

Why?  Why should there be a strategy which is 100% undefeatable except for "do what the other guy does, but build more of it?"  Remember, an attacker doesn't need to actually use their agents to harm the defender through attrition warfare from upkeep... and since there's no way to kill wizzies, that war of attrition can keep going on forever.  How many good strategy games exist in which you can spam one unit and an opponent has absolutely no tool to stop that unit except to counter it with their own equivalent?  Besides, how many times do we hear people say that if an opper really wants to hit a banker, they'll probably end up succeeding?

That's not a defense.  In many cases, it's hardly strategy.