"Easily sustained cruising speed" is going to be much different for different riders.
With my history of leg injuries, I probably need more help than others.
In particular, my knees cannot handle a lot of pressure, so low input torque is the limiting factor for my riding.
And since aerodynamic drag increases at the square of velocity (F=mv^2) I need more help to maintain speed as the velocity increases.
I understand completely about varying levels of effort we are each and all of us capable of. That can vary and change with flare-ups of this condition or that as we age.
At 70 I am dealing with some significant health challenges, including widow-maker Mi in 2017 that should have killed be but got me started riding an ebike. Then over the past year, chemo, radiation and major surgery to remove a malignant sarcoma discovered under my left quad, pressing against the femur, with over 4 months using a walker, then crutches then cane with PT and rehab.
My ability to ride has been all over the map this past year. I am still on chemo which has me slightly anemic, but able to ride pretty regularly. I often start out lethargic, not sure of my staying power. Then after five miles or so, my blood gets up, the endorphins kick in, I get the wind in my face and I find energy I didn't know I had. Rides typically end feeling stronger and more alive than when they began. I can't begin to express how important being able to ride through all of this has been to me.
The Cannondale Topstone Neo Carbon 3 I bought back in December was a personal reward to self for rehabbing my left leg. The bike has been a real revelation as it is so much lighter, only 39 lbs. with a Bosch speed motor and a 500 watt power tube. Designed as a gravel bike, it has flared, shallow, drop handlebars and a modified aero position that I can be comfortable in for miles and is far more aero than on my Trek Allant or Delite Mountain with their flat bars, fenders, thicker frames, etc..
I am going about 12% faster and further in Eco on this bike than I can in Tour with the same amount of effort on the Trek and more like EMTB on the Delite. Last week I did a 40 mile ride and was apprehensive I could do it in Eco or Tour so I did it all in Sport, averaging around 17mph, mostly flat rural roads. I finished the ride with 32% of the battery remaining. I would get no more than 32 miles in EMTB on the Delite.
So yes your are right about aerodynamic drag being a huge factor in battery use. I also think that we ebikers tend to overlook the cost in range that our heavier ebike impose as well. Yes the motor can take up the slack when we keep adding heavy gear to our bikes but it does cost more in battery range.
