It's a tough task for BMW ...
Audi / VAG has been pouring tons of money into D5 A8 development. The new A8 is expected to outshine S-class & 7er in "advanced tech", while the new Phaeton is planned to be on the 7er / S-class level. Very ambitious goal!
Audi is determined to trash 7er & S-class when it hurts the most: BMW in driving dynamics & all-roundness, and MB in comfort & composed ride.
S-class is already here. As it is. OK, new gearbox is coming & new engines too. To give S-class a bit more edge.
7er is coming ... with advanced "intelligent weight reduction" with some CFRP help as well ... New generation of modular engines. Some new ConnectedDrive tech. But nothing really revolutionary. Comfort is better than in F01 but still not on par with S-class. Nor is the luxury opulent cabin ambient. But the car drives much better than F01, better than current rivals.
But as said ... industry rumor has it Audi is coming with a real winner here. The product will be great apparently but will Audi be able to outshine BMW & MB when it comes to marketing & sales. A8 is still lagging behind very much. But insiders say a lot of money will also be invested in marketing & sales by Audi. So the gap is expected to shrink even more.
I guess Piech plays an "all or nothing" game ... and all his, VAG's & Audi's stakes are on "all".
The rumor has it the R&D + marketing + sales costs for the new A8 / Phaeton will exceed the ones of 7er 6 S-class ... 

 COMBINED!!! So that makes you think Audi is dead serious with this project & goal to trash 7er & S-class.