Personally, I would stick with 75,000 miles or 5 years. Rubber deteriorates with age, and if the car has a 15 year old belt with only 80,000 miles on it, it's probably about to snap! Mileage is not the only factor.
Problem is Audi and everyone are trying to make their cars "look" cheap to maintain. So they put these high mileage belt changes, "sealed for life" gearbox oils, diff oils and even power steering fluids! Then do long life servicing.
Yeah, on that kind of regime the car will last well past warranty, (engine warranty is only 60,000 miles anyway IIRC), but when it goes pop later down the line it's not their problem, so they don't care.
If you want to look after your car, change out those "sealed for life" fluids once in a while and change your timing belt & water pump every 75,000 miles or 5 years, whatever comes first.