That is to take the view that it's a "needs-driven" decision to use an e-bike, not one of choice for the utility benifit, which is the reason I got one .. seeing as I gave up my gas-guzzling polluting car too. It seems that the perception of e-biking here is too focused on it being something to help those who could not otherwise cycle, not to address it in a proper rounded way from all sides.
You chose to use a bicycle for it's utility benefit, and if you feel you need some assistance the law provides for that. That is it's purpose, intentionally a needs driven law. No law is "properly rounded from all sides", that expression is without meaning since any law has to be specific in purpose to be understood and administered. "Properly rounded from all sides" means suiting everyone, so no law at all since everyone's requirements covers everything.
What you are asking for in truth is another motor cycle class, since it is a fact that most of the world's cycling is done at speeds well below 15 mph, as it always was in Britain before cycling receded into a largely sport based pursuit.
E-bike law doesn't provide for the common sport cycling speeds for obvious reasons. If you can cycle routinely at much higher speeds than 15 mph you don't need assistance. If you can't but can cycle at more normal utility bike speeds, there's no reason to assist you to go faster. If you struggle to maintain utility cycling progress, the law provides for that with some power to enable you to cycle as well.
Anyone wanting to have higher speed assistance needs to comply with the laws appropriate to that, as the Germans and Swiss do for example with their high speed classes. It's also worth bearing in mind that higher speeds tend to need more than 250 watts rating, as the influx of 350 and 500 watt motors in the German high speed class e-bikes shows, making them even further from bicycles.
That brings us full circle to the original point of the law, retaining the e-bike as only a bicycle and not in any way a motor vehicle.
.