If you're going to change the factory intervals, you need to understand all of the reasons for each specific interval, and think it through like an engineer. If you're going to do this, you'd better be real sure that you know what you are doing- that you fully understand the considerations the VW engineers used to make the intervals what they are, and know why a different interval is superior in your specific circumstances.
For example, how exactly does the physical process of aging/wear work for each specific part? For example, one could assume that an engine air filter replacement is based on filtering out a certain amount of physical debris, and should be more tied to mileage than age. However, a serpentine belt will oxidize from ozone when just sitting there, so should not be extended with low mileage.
In all cases- if you can objectively measure the actual condition and wear of the affected system and replace based on that, it would be an all around superior way to choose correct intervals based on your specific use. For example, one can remove a small amount of engine oil and send it for oil analysis to determine if the oil is actually still in spec or not.
In short, if you have to ask this question on a forum, you should just follow the factory recommendations.