Diffs and gear boxes of the greatest quality, lubricated with the best of lubricants, always operated within spec would last a very, very long time.
All three of those stars aligning over decades? Yeah nah. I'm told changing the oil in your automatic transmission makes a hell of a difference. But I've only ever driven manual or electric.
Automatic transmissions are far different than closed systems like gear cases.
How many 100-150 year old tractors have gear oil in the belly that has never been changed?
The tolerances are of a Swiss watche, they don’t need to be. They just need to work, not mesh like a F1 car. Fuck I disassembled a windmill the other day. It hadn’t had oil in the gear cases for probably 30-40 years. All parts worked. Without issue. I turned it upside down and gear dust came out. Lots of it. More than you think. But because the White engineer on aermotor knew the conditions they would be in. The gears are heavy properly case metal. If it was just cleaned and oiled all would go back to work without fail for another 100’years. This one specifically was from 1920.