I think the current 15mph for E bikes is just right. Let's be honest a "good" cyclist will usually only average 17/18 mph on a road bike out on a good ride out. Most of us will average much less whilst the super fit will average 20 mph. If you would to increase the legal set speeds of an ebike to 20 mph you would no doubt get people calling them to be upped to 25mph. Where does the line be drawn on this ?
Granted all bikes can be ridden much faster than this, but its usually in short bursts. Hurtling down the Horseshoe Pass i touched 45mph. It felt like a moment of madness as my road bike felt very twitchy, and looking at those thin tyres didn't inspire confidence. Good fun whilst it lasted, but I wouldn't want to do that all the time. 15mph is a good speed to cycle along and its in keeping with the majority of most average cyclists, so why change it. The whole point of an E bike is to make cycling easier for people who struggle to use a normal bike, or for people who live in particularly hilly areas. I cycle with a chap (who's been a life long cyclist) and is now in his late 80s. He's got a very nice road going Ebike and wouldn't be able to come out and enjoy is life long passion of cycling, if it wasn't for his Ebike.
My dad is also in the same position, in that we can both enjoy cycling together, in his aging years, were we can enjoy some quality time together.
In essence, an Ebike should always be classed as an electric assisted bike which is in the same average speed range of a normal bike. If you want a faster electric bike, then they should be registered ones like the Super Soco, or Lexmoto cypher offerings.
Just my two pence worth👍