The only numbers you need to know.. Audi A8/S8 sales down over 30% in the US for 2016 going back to 2013, about when the Model S started to become more available:
Audi A8 / S8 US car sales figures
We are only talking about 6300 down to 4100 cars per year, but at the same time the Model S went from just 2650 in 2012 to nearly 29,000 in 2016:
Monthly Plug-In Sales Scorecard
Audi is stuck in the past and this is just another example of how wrong they are about the future of cars.
A4 still does well, but that will end soon with the model 3. I feel bad for Audi, they seem to be a decent little brother to BMW and Daimler, but sadly they are all lost causes at this point. I assume VW will end up owning them all as they go bankrupt over the next few years. Of course VW will also be much smaller and weakened by legacy thinking, but if one has to survive, I am going to go with the biggest.