Fully a selfish request as I’m currently experiencing the situation, nonetheless I think it’s not an unfair one.
I have unlocked everything from the silver boxes which is unique and volume limited (squads, soldiers and vehicles). Once that happens, the odds (1%, 2% & 3% respectively) are moved fully silver unlock chance (+6%).
So far as I’m concerned, I only have a 3% chance to roll something interesting (weapon category) at this point. In the next such event, I think it would generally be fair to transfer 50% of the odds to other such categories and event weapons and the remaining 50% to silver as is.
So if I for instance if I have all event squads (1%), then the odds of weapons, vehicles and unique soldiers goes up by 0.166% each and silver goes up by 0.5%.
This would mean that the value of the box doesn’t drop as dramatically as in my case, where I went from 9% chance to 3% chance (-66%).