My 2 cents...A lot of opinions on this thread. What I do know is:
1. Manufactures do extensive testing and don't randomly arrive at these numbers. They are data driven - for the most part.
2. Manufacturers also provide warranties (4 years for example) so at least initially, there is some incentive for the car to not breakdown beyond some variance before the warranty period. Otherwise, this would affect their financials.
3. Oil does degrade based on use i.e. number of miles and also over time.
4. Most modern cars have sensors and the actual mileage for oil change may change based on type of driving, miles, oil quality, etc. Having said that, I do NOT know this specific to Range Rover.
I also feel that 21k is a lot BUT that is based on previous cars I've owned and have not compared the type of engine, the oil used, the engine specs, etc. etc. My point is - it's is easy to simplify this into car X requires 10K therefore all cars should be changed at 10k. The reality is, it's A LOT more complicated.
My dealer recommended annual change but I'm leaning on going with the Manufacturer and NOT the dealer recommendation.