what are engineering reasons for ZF 8AT vs. DSG S-Tronic 7 speed?

[whobodym]

As a car enthusiast, owner of 2014 A5 2.0T 8AT conventional torque converter automatic (USA model), and an engineer myself (but aerospace, not automotive) I am very curious about Audi's evolving automatic transmission philosophy. When my car was new, the A4/A5 2.0T automatic in quattro models in USA was the ZF 8-speed, and the faster more expensive S4/S5 models came with 7 speed dual-clutch S-Tronic transmissions. Now, with the new B9 generation, the opposite is true, 2.0Ts are 7-speed dual-clutch and S5s are 8-speed ZF automatic. Why? Has the technology or cost or performance of the one changed relative to the other? Or is it just business thinking like the contract with ZF, the size of respective factory facilities, etc.? I do know that somewhat counter-intuitively, the gear ratio spread of my 8-speed is actually somewhat narrower than that of the 7-speed in the 2019 version of my car - I get 1500 rev/mile in my 2014 8AT in 8th gear while I think a new 7DSG is ~1350 rev/mile, while 1st gears are quite similar in both. How are durability and dependability comparisons of 8AT vs. 7DSG looking? Those more informed or intelligent than myself, please chime in.

[ellwood]

durability wise, the ZF8HP has been proven to be extremely reliable across a broad range of applications. mechatronic issues on the DSG boxes are reported fairly frequently on this board. additionally, the 8HP performs sequential gear changes up or down (and many other shift sequences) by unlocking one brake/clutch and locking another, so shifts are comparably quick to a dual clutch. further, the torque converter on the 8HP operates smoother in low speed applications (e.g., start/stop traffic)