Anyone with eyes, a memory and an objective mind can see for themselves that bearings fail due to water ingress, followed by corrosion which pitts the surfaces and rapid wear to failure follows.
It’s pointless to check the manufacturers instructions. If any one has ever done this, they’re so generic as to be pretty useless. ‘service life may be fore-shortened if used in dirty conditions or the presence of contaminants’ is a pretty good example.
Fact is, the bearings used on bikes are co-opted from industrial machinery and are designed to run at significantly higher RPM in cleaner environments. Even the way bearing specs are quoted in catalogues shows they’re not design for the way they are used on MTBs.
If you’re not a ham fisted buffoon it’s easy to use a Stanley blade and lever a seal out from its outside edge. Pack with decent grease and then press the seal down. If a bearing is packed with grease, there is less space for water to occupy. Personally, I like finish line ceramic. It’s not the thickest, but it seems to add a rust resident cutting to the the bearings, which gives you a bit more grace with you re-greasing interval. I tried RnR super web, it’s very sticky, but not actually very good with water. It looks to me to just be a higher soap content lithium grease.