Changing the CVT oil at a certain mileage is a good idea as a guideline but it should also depend on the style of the driver. If you are aggressive on the throttle, then, the CVT gearbox has to work harder and puts more stress on the moving parts especially on the belt. Just like a manual gearbox with the clutch plates, if you are aggressive with hard acceleration, your clutch wears out faster or rather, the steel belt on the CVT box. The main issue of gearboxes is after warranty if you plan to continue driving the same car. Changing the CVT oil sooner can mean extended lifespan for the gearbox after warranty. Which is cheaper, changing the oil or gearbox?
Car manufacturers have basic guidelines but also need to ensure that it is not too expensive to maintain a car. Let me give you an example, BMW's autoboxes are maintenance free, no need to change the oil, but at 150K, you are supposed to change the whole gearbox. Ouch!