Porsche / Audi are selling a lot more cars then they were 10-15 years ago. I think they will be okay. Chinese brands will cut into all establish automaker sales, but the German cars have strong brands that symbolize luxury. Look at Land Rover. There are cars are completely unreliable and they are selling more than ever at increasingly ludicrous prices.
I agree with you, but luxury car manufactures largely sell leases. So they are designing their cars for that market.