Some pretty unrealistic expectations for a $20 BB cartridge I`m afraid. A better explaination of `sealed` in this case would be ` not servicable`.
I`m sure that some mtb riders abuse their BB and tell about it but I doubt they `get away with it`. Replacing Shimano UN 53/54 cartridges is a pretty common occurance in all the shops I`ve worked in and the reason is always the same - water infiltration.
Most common on the crank side. Personally I`d suggest a teardown just to dry out the water thats probably still inside the frame. It can`t get out on its own. But thats another thing professional riders do too - tear down their bikes a lot.