IDK, I'd think, more speeds would equal more hunting for the right gear, more heat and more expensive when it fails. People don't keep vehicles long enough anymore to know, and the long term tests that manufacturers do aren't real world usage so its hard to say how things will go but they think adding more gears gives them better fuel economy but it don't, its just a more expensive failure item. I bet they'd change their minds real fast if they had to shift through all those gears manually LOL.
No, you are wrong. The idea of an auto isn't to pick a gear and sit in it. Autos these days generally try to run up to top gear ASAP while keeping the engine as close to the peak of it's power band as possible. I've watched on a scanner as a 6 speed auto ran itself up to 6th gear by the time I was doing 35 MPH.
So a 10 speed auto isn't going to hunt in the usual manner a 4 or 5 speed would.
Also, this isn't your granddaddy's auto. Like I said, with modern electronic controls on them you can turn what was once a 4 speed trans into a 10 speed with more software changes than hardware changes. And most of those hardware changes would be in the valve body.
A 10 speed manual is a little untenable in the average passenger vehicle. Unlike an auto where you can stack ratios through the gear sets, which can each produce multiple ratios, included several OD ratios, to tailor things, in a manual you have to add a physical gear (two actually) for each new ratio. You also need a new fork and rail for every two gears. This means a 10 seed manual will be both very long and very wide.
Tractor Trailers get around this with their "18 speed" transmissions the same way Wheel Horse built "8 speed" mowers in the 80s. They put a reduction splitter on, which is essentially a two-speed transfer case with no front output, so they get to count each gear twice. A far more complex and expensive process.