That ^2 is a load-bearing exponent which changed my entire perception of safety.
With the oil crisis making it a good time to buy an ebike, I see the same problem popping up on bike forums that I faced when making my original purchase. At least here there's the option between a Class 2 (32kmh max, usable on paved trails) or a Class 3 (45kmh max, limited to roads). When I bought my first bike I got a Class 3 because I'm getting 40%~ more speed for the same price. I could arrive at my destination 40% faster than I do on my cargo bike.
Assuming a rider+bike weight of 100kg and converting the two to m/s: 8.89m/s, 12.5m/s-
Class 2- 0.5(100)(8.89^2)= 3950 Joules of energy. That's what you experience when crashing at max speed, the equivalent of falling off the roof of a building with one floor.
Class 3- 0.5โ(100)(12.5^2)=7,810 J.
Despite the speed only increasing by 40%~, the kinetic energy you experience in that crash almost doubles. The energy your brakes have to absorb to stop, and with that the stopping distance and wear, almost doubles. Your risk of fatality jumps significantly. You are now jumping off the tallest house in your neighbourhood and that is the level of injury you can expect from the slightest mistake.
If you or anyone you know is tempted to get a hyperbike because it goes much faster than a cargo bike, especially beyond the Class 3 speeds that I don't even want to think about the injuries from, that impact force needs to be a primary consideration. I am confident that with proper safety gear I have a 90% chance to survive a crash with injuries that won't last a lifetime. If my cargo bike was Class 3 instead of Class 2, I could not maintain that same level of confidence. It's such a minor difference when you are looking at generic speed numbers but you will crash and that is the standard you need to prepare for.
Buy a Class 2 and maximise your weight-bearing utility over speed. Slow is smooth and smooth is fast.
Safety is a big factor but one that is pretty easy to plan for for those who want the speed. Like if you wanna go 60km/h plus on what is basically an electric dirtbike you can just wear a full face dirtbike helmet and gear. It's still a very good point you make since I don't wanna put on all that gear for a quick trip to get groceries.
I think for me one of the other huge factors in taking it slow on a bike or with personal transport in general is energy. The energy required to overcome wind resistance also scales with velocity squared. So going 1.4x as fast should require about double the battery capacity. Actually more than double, since the slower you go, the more significant your own pedal power is. I don't have an e-bike yet, but even without the battery, I can ride so much farther if I take it easy and average like... 15km/h instead of trying for 20.