I can’t wait for pedal assist ebikes to become more affordable. It opens up biking as an option for a lot more journeys, not just for lower-fitness people, but in regions where it’s hilly.
For where I live in Los Angeles, the places I bike to are a couple of miles away (or close enough that walking makes more sense), but uphill all the way. Having pedal assist would mean that I don’t arrive sweaty with tired legs. I don’t need to go 25mph, just 10mph with less effort.
Second type is torque sensing. This is quite a bit more expensive (an additional couple hundred, plus higher installation skills if you are converting yourself).
Another note is that most assists don't sense hills, so you will have to manually adjust the assist level when going up hill. So at that point, using a throttle-only setup, with a throttle lock (hold throttle in a position for 2 seconds and it keeps it at that setting) works really well. In that case it gives you the sensation of having a good wind at your back helping you out.
The primary cost can be the battery -- anywhere from 200 to 1000, depending on capacity and quality. Second is the motor, 200 - 500. Some come with kits that give you simple throttle control, or you may want to add an advanced ebike computer kit (like I mentioned in the previous comment).
All told, I'm about 1200 in my ebike, plus the bike itself.
So the argument is that it isn't bad when a small percentage of cyclists hit 25 - 30 mph, but when all of them are able to that is where the issue is.
My personal feeling, however, is that especially the models that are pedal assist (where you put some effort into it, and it multiples your effort) is a good way to get otherwise low-fitness people to bike ride more. The more cyclists, the more push for bicycle friendly infrastructure. And people still get exercise on them, more so than in a car.
But regardless of powered vs unpowered, anyone that rides a bicycle fast by pedestrians and dogs is doing it wrong. On an ebike, it should be no effort to slow down to 5 - 8 mph, then speed back up after passing someone.