As I kept saying before, they need to allow the teams a "pool" of gearboxes like they do with the engines - let the teams decide where and when to use them and give them the flexibility of swapping in a gearbox that hasn't done as much mileage.
It still keeps the costs down by limiting the number of gearboxes, but makes it so teams are restricted to using gearboxes for certain events.
Indycar could also learn from this too, seeing as they like to throw silly penalties around for "illegal" engine changes.
The point isn't to penalise the teams for changing an engine or gearbox early. The point is to limit the number of engines/gearboxes they go through to reduce costs..rather than them using fresh parts every race.