The problem is the difference between riding on the roads and on cycle paths. At the moment, we can use both. 15 mph on the road is probably too slow, but 20 mph on a cycle path is definitely too fast. Not for the cyclists. I'm thinking about people with dogs on long leads and with kids not looking where they're going, let alone all the ones tacking towards you while their heads are looking at their phones.
If by 'Cycle Path', you mean a lane reserved on the 'road' for bikes, then you'll ride to keep up with the traffic.
If by 'cycle path', you are refering to a path 'on the pavement', or a path through woodland, then 15mph is likely to be too fast with people around you.
Put another way, just because a car can go120 mph doesnt mean it is going to be driven at its maximum speed. You drive or ride as appropriate to your surroundings. eg common sense.
The other factor to limiting ebike speeds would be protective gear. Helmets, jackets, trousers, gloves. So far i've not seen this point made with regards to ebike usage.
