Even without the box I think you're underestimating how damaging water is. If you're experience a flood it's never distilled H20, sometimes it's sewage and that's just awful, but even if it's storm water or a broken water main the water isn't the difficult portion (though that alone can lead to all sorts of mold issues) it's the sediment. If that sediment is from brown water there are obvious biological hazards which may lead to destruction being the only economical resolution, but even if it's just mud and sand that forces a huge expenditure to actually clean the products and if there's a signficantly misaligned pH it may damage products that you otherwise think of as water resistant.
Given this was a set of full star wars legos with decades of age a lot of those bricks are already going to have degraded somewhat and technix style components are likely to be significantly damaged from internal sediment accumulation. If you drop your water proofed water in a stagnant pond for three weeks it's likely that the internal seal will hold up and protect the delicate components but you'll probably need someone to pull the glue or other sealant out and replace it as well as going over the exterior surface with cleaning solutions to get it back to the quality it was in before being submerged - and flooding is rarely an instantaneous affair.
I wouldn't underestimate just how damaging to goods storage can be - and if you're doing it at scale you're going to be paying that cost constantly just as a percentage of value stored.