The DSG is simply superior over a conventional manual in a car like the RS5. The last RS car with a manual transmission was the TTRS. When Quattro Gmbh introduced the TTRS with S-tronic, manual transmission sales tanked. That was the point where they decided to no longer offer RS cars with manual transmissions anywhere. I've been a long time stick shift driver and there was a time where I didn't even consider anything that didn't come with a stick. Now, I won't go back.
The important thing to know is that the S-tronic in the RS5 is track tuned and it has a true manual mode, meaning it will never upshift automatically in manual mode and even let the engine bounce off the rev limiter. It will only downshift automatically to prevent the engine from stalling. The high revving nature of the RS5 engine really calls for this kind of transmission. It takes only a split second to put the engine into its sweet spot vs. fumbling with a stick shift.
The reality is that for spirited driving and driving on the track, the DSG outshifts anybody with a manual transmission and in daily traffic you have the convenience of the fully automatic mode. It's the best of both worlds.
EDIT: I forgot to answer your question. You can't switch the DSG for a manual transmission in the RS5. The DSG is integral to the car, because it is a requirement for the 6th generation quattro system that was introduced with the RS5. At the core of this system is the new self-locking crown-gear center differential and only in the RS5 brake-based 4-wheel torque vectoring in addition to the active rear axle torque vectoring courtesy of the sport differential. Cars with manual transmissions such as the S4 and S5 are stuck with the previous generation quattro system, unless you opt for the S-tronic, but they don't get the 4-wheel torque vectoring, only the rear sport differential.
There is no question about the superiority of the S-tronic.. The RS 5 and the R8 are the only models which will hold the rev limiter and not force an upshift. I too used to be one who would only drive a manual, but that was before all of these awesome tiptronics and S-tronics came along.
When the only options were slow shifting 3 or 4 speed slushboxes, yeah, a manual was the only way to go.
Now? I'd rather have a quick shifting auto or dual clutch.
I'll be honest I didn't consider the RS 7 in this discussion because it is not a dual clutch box, but I recall that it will also hold redline in M mode... not that you need to in that car for any reason, better power is lower. =)
I do not necessarily want to speak for the original poster here, but there are a few of us that while acknowledging the superiority of a DSG transmission for providing quick shifts, still prefer a true old fashioned row-yer-own manual transmission. Personally, I am at a point now that I would be willing to pay for the option of a manual if that's what it takes to get one (which at a very low take rate on cars that offer a manual, may be the only way to save them from extinction)
I would never pay extra for a manual transmission at this stage for the main reason that they are now simply being pulled from the parts bin and they often need to be mated to older outdated drivetrain technology. Mfgs are no longer investing into making their manual transmissions compatible with their latest drivetrains. Audi is a good example. The 6th generation quattro system was built around the S-tronic and then later adapted to the 8-speed tip for the S8 and RS6/7. The other part that irks me is that the latest crop of manual transmissions aren't so manual anymore. They are now all auto blipping the throttle on downshifts. Might as well go with a DSG at that point. The tides have turned. Now that the R8 has the S-tronic, people who previously recommended to opt for the manual transmission are now recommending the S-tronic. The A3 sedan, a volume car designed for the US market is DSG only in the US. Audi couldn't make the numbers work to bring the 6mt over at least for now. It will keep going that way, and once we get off the internal combustion engine, the need for shifting gears will disappear altogether. Cars like the Tesla S have "two" gears. One forward and one reverse.
While the DSG does provide a quicker shift, it is far from perfect. Forums are liter with threads and posts about hick ups with the system as it can be "confused" and caught in between shifts. This has happen to me several times during the past 4 years of ownership and had I not caught it quickly, the result would have been disastrous.
Had manual gearbox been available, I would have certainly be more than happy to pay extra to get it. And the lack of it is the main reason that is holding me back from ordering a RS5. DSG may be quicker around the track, but the lack of involvement ruins the pleasure of driving and makes it a much more sterile experience than the good old fashion manual.