Not being involved with this in any way, ie I don't currently have any cars on flexible servicing or still on a prepaid "service plan", I knew that it was the "way things are done within VW Group world", when my wife bought a new Polo in 2015 and purchased a 3 year service plan - ie that the car would get handed over new with the servicing option being set at "fixed" - if we had confirmed that this car was going to be used in a way that involved high miles/year, then it would have been handed over left set as per the factory default at that time, ie "flexible servicing", and I'd doubt that they would have tried to sell us a "service plan" which I have always considered to be a "(fixed) service plan".
As to what the Audi garage were thinking about when they sold my "bought used" 2011 Audi S4 when new, it was still set as being "variable" when I bought it used at 2.5 years and 12,500 miles - using the established VW Group way of "talking to the customer at the point of sale" to work out if "variable" or "fixed" best suited their intended use of a purchase
Chauffeur-driven cars does that seem to have been done. That car, for its first 2.5 years did end up getting serviced when it had run out of time on the flexible service strategy.
It does seem to me, to make a lot of sense to have separate terms & conditions and maybe pricing for a fixed (time) term service plan covering "fixed" servicing and a fixed term (time) OR (miles) plan covering "flexible" servicing. Obviously too tricky for VW Group service points to work with.
One size fits all?