I've driven 6 speed manuals too, but I don't have any illusions that I'm doing so optimally. And never mind other drivers on the road. Many people who drive cars with that many gears skip a gear here and there. Also, speed is not the only criterion for when to shift. It has to do also with your torque demands at the time.
Just like humans can't hope to beat a computer at adjusting the spark advance or the fuel/air mixture, or for that matter a manual choke, all tasks humans had to do in the days of the Model T Ford, humans equally are unlikely to manage transmissions with many gears properly. That is, well enough to beat a computer.
My automatic, VW Golf (gen 6) TDI with DSG will shift to 6th, and be quite comfortable, somewhere around 36 mph if I let up on the accelerator. It will eccelerate from there without downshifting with the further application of torque. Of course the car gets best fuel economy in the range of 35 to 45 mph. As the speed gos up from there it's mostly a question of air resistance going up in proportion to the square of the speed. I wish RPM, Load and Fuel Consumption charts were available for auto engines as they are for marine engines.
I think you would soon manage a 7-gear manual. My last three cars have had 6 gears and I find managing that a no brainer. I guess it depends on where the 7th gear is: my current car is best with 30mph (4th), 40mph (5th), 50mph (6th), so there is room for a 7th (60mph) and an 8th (70mph). And at that rate, most British drivers would find room for a 9th and many a 10th! :-)
I think you'll find that this isn't true anymore. Automatics now have more gears than manuals very often, and they have had lock-up torque converters for many years now. These days, 6, 7, or 8-speed automatics are becoming commonplace -- more ratios than a manual transmission driver could hope to use effectively. This means that the car operates in a more efficient regime more of the time, and there's no wasted energy in a slipping torque converter, once the gear has been selected and settled down. So really, there aren't many downsides anymore. Car companies that do keep manuals in their inventory do so more because of consumer demand than out of necessity.
Companies like Mercedes Benz are trying to wean drivers off of manuals, specifically because it's easier to meet tough new fuel economy requirements with automatics, these days. When manuals are sold in the US, at any rate, in some cases the transmission automatically "urges" the driver to shift (with a light on the instrument panel), and even to shift to a specific gear. And it's logical. Even something as simple as the manual choke had to disappear years ago, when catalytic converters were first introduced. Why? Because humans are too sloppy. A manual choke would kill a catlalytic converter in short order. Ditto the old mechanical carburators, when the three-way catalytic converters were introduced.
So, it should not be surprising if one of the few remaining manual controls (aside from steering, braking, and throttle) would disappear in time. Not to mention, automatics have also become more maintenance-free than manuals.
I think it's instructive to see how many start-stop car drivers know how to, and habitually defeat, that feature. I'm sure that software control of the feature will improve in time, across all brands, to where drivers won't feel obliged to manually override it. It will also be interesting to see whether, long term, they create maintenance surprises. (Probably something that will vary by make and model.)
I'd assume that even well designed automatics based on torque converters would be more efficient than most manuals driven by people who either aren't paying attention or who haven't been taught to drive efficiently.
As you say, most modern multi-gear automated manuals should be even better as they don't suffer the losses in the oil-based torque convereters, and I'm not sure I could pick the best option from 7 gears anyway!