The caps part of your argument I get, if thats your style. I dislike truck caps, and feel that if folks want to put a cap on a truck, they should have got a large SUV instead. But thats just my opinion, to each their own.
However, the corrosion comment does not apply. The only parts that are metal in there are the hinges, the latch, and a few bolts. So sure, if you filled them with water, and left it in there for weeks or months without opening/draining, sure, the hinges/latch/etc. would probably start to get rusty. However, the boxes themselves are plastic/composite, not metal, so they do not corrode. Even their tops, although painted to match the rest of the truck, are plastic.
They also have drain plugs at the bottom, so if water does get in (for example, if you throw drinks into them with some ice), you can drain them.
Water should not get into them through the door seals. Once properly adjusted, the only time I could get water into my 2014's was when I used a high pressure wand on them 18 inches or so away, blasting directly at the seam/seal. I have not tested my 2020 yet, but they look better sealed than even my 2014. We shall see.