Bit confused here. They are stopping Defender production in 2016 because it doesn't meet 2020 emission regulations. But VW can continue production of diesel vehicles that do not meet the 2015 emission standards. Doesn't compute. Well not without a nifty bit of naughty software ...
There you have it in a nut shell, it has nothing to do with EU emissions and everything to do with the willingness to update it with modern engines and safety standards.
Plus the VWs can meet the regulations, but will either have less power or need more AdBlue, increasing running costs.