I can only quote from my own experience here and give you my thoughts why 15.6 mph is too low, I know you can pedal above this speed but old age and infirmity limit this.
When the Avocet computer appeared in the early eighties,we used it on club runs to keep the speed about 18 mph this was a good group riding speed and IMO still is today, most riders I rode with in those days can still keep these speeds up today, but one or two have dropped to slower groups which is perfectly natural when you cant keep up, but with modern technology we have a solution that would enable them to still ride with their old mates, this requires an higher assisted speed than 15.6 mph above which they would struggle.
In my experience (I know it wont be everyone's) most people ride between 16 to 20 mph average, just above the cut off for ebikes, so a 20 mph cut off makes sense to me.
Lifting the limit won't stop anybody on a normal bike cycling, but it will enhance a lot of peoples enjoyment of ebike's.
Lets embrace the technology and make it work for all cyclists.
I don't own an ebike, but never say never, I am fortunate enough to still be able to average 18 mph on a normal bike.