I have other sealed bearings this low and have now statistical evidence of them being gone after a year in my conditions. This has been stopped by going to ceramic. All the BBs I have, so far, used had cups that provided additional and likely more thorough external sealing than BBs themselves. The cups do not need to meet any, excessively demanding, standard of thickness. However, I do remember cups that gave no sealing, which I avoided. My experience is from cartridge BBs, but when I look at sample cup and cone BBs on-line, they seem to have at least potential for better sealing than cartridge bearings alone.

In any case, there must be a good reason why slotted shells have departed into the past, together with U-brakes and such.