You're missing the point arguing about flexing side walls - it's about the knobs.
This does not matter with an ICE engine, or on a car, where the amount of fuel it's burning is relatively flattened. On an electric motorcycle, you're dealing with the amount of energy contained in one third of a gallon of gas, with a power train that will draw more or less exactly as much current as the physical load demands (to reach what the throttle asks for).
Because of this, certain aspects are magnified, and while streamlining is huge, and tire pressure makes a big difference, the tire tread itself is also in there and yields a small benefit. Regarding safety, I'm quite sure that mine and others' tire choices mentioned here are safer than most S/SR riders' tires.
Your idea of "less thread" would make sense if it were somehow on the same scale. I remember noticing when they came out that the 15DS (same for 16DSR) had a stock tire choice that was too knobby for the street, leading test riders to be uncomfortable the way the tread "walked" around a bit. I just mean that backing off of this for a 90/10 tire yields a little mileage benefit, and it's not unsafe at all if your riding really is a 90/10 mix.
Feel free to posture more about this strawman version of this argument, but I think I've clarified my interpretation as well as I can. I want to make sure that people reading this thread don't think that a 90/10 tire is somehow a crazy dangerous choice to make for a DS, which you seem to be intent on convincing someone of.