Are wrenches single-use, or are you allocating individual attackers per-citizen? In the case of a successful wrench attacker, how are the stolen funds recovered?
To be maximally effective you'd need to mobilize enough attackers and enough windowless white vans to reach the entire populace within probably a day, so let's say 3 million attackers and 3 million white vans. That's when things start getting more expensive than the sql query.
i would guess that percentage of people with wallets would be very small and a relative small team could with a relative small number of white vans and wrenches could get every single one in relative short time..
and for countries that heavily monitor internet usage, like china, i bet they could have a list with pretty good assumptions on who has crypto that they can target for further investigation before sending the wrenched goons..