No, seriously, if you want people to get out of car and commute from, lets say for argument 10 miles each way. They will invariably want to arrive non sweaty and in good time.
20 mph is not unreasonable or excessive, it may not agree with your being overtaken effortlessly attitude. It aligns with the 20mph suburban/city speeds limits. 15mph cut-off, means an ave of around 13mph to stay under and receive full assist. 20mph cut-off , ave 17-18mph.
It means a journey of 10 miles if cut from 46mins to just over 30 mins. it may not sound a lot, but people's perception of travelling and time can be critical, hell UK government spending hundreds of billions on HS2 to reduce travelling time by 45 mins. If we are to encourage more folk to use less polluting transport, every bit of encouragement should be used. Similar commute time via a bike compared to a car is crucial in attracting the switch to cycling.
Speed pedelecs are common in Europe for their speed and
the easy legislative process of acquiring the necessary documentation. I suspect the low costs too make them attractive.
Imagine replacing thousands of cars going into cities for speed pedelecs or 20mph restricted ebikes, far less pollution, accidents would be fewer and much, much less severe compared with vehicle/cyclists collisions. We all know cyclists don't like to fall or crash, because it hurts, cars drivers are isolated from these sensations.
I know folk who have fast e bikes, they say it takes a fair bit of effort to maintain a 20mph ave, due to drag resistance, but up to 17-19mph the effort is almost like a gentle pootle in effort.