Hi Gareth. The same debate is often on various marque forums, not least the MB ones. I think sealed for life refers to a plastic plug inserted in what would be the dipstick tube. What would happen if the sump on the gearbox started to leak? ....Why do Audi sell the ATF , gaskets and filters?. The price of a fluid and filter change is listed under the price list from Audi dealers.
Mercedes offer a fluid/filter change on ZF boxes as do several other makers who use exactly the same box.
I think its the same with the long life service. I think this is to attract fleet buyers to keep the cost of servicing down. I will keep an open mind.
Thank you again.