Posted on behalf of @Crackle who just missed the lock prior to hitting the post button.
Having thought about this, I don't see the a reason why the limit shouldn't be raised. Will it be abused, are there people who would be dangerous on faster bikes, sure I bet there are in the same way lots of other things are abused, we don't legislate to the lowest common denominator though but we do have laws to catch people flouting legislation, so I generally think pointing to such occurrences is moot.
I also think that it's a bit of a pie chart as to who uses e-bikes and I don't think that the demographic is quite the same. Potentially e-bikes as cycle to work initiatives could be getting people out of cars and off the roads, this is a good thing, as is getting people generally out doing some form of exercise , even if it's lighter than what we as 'ordinary' cyclists would do.
Whether it should be 20mph is I suppose, up to debate, 15.6 is rather arbitrary, perhaps some compromise where the limit is a little higher with an ability to boost speed for short intervals of 30 seconds or so for getting out of situations which requires that extra speed, such as pulling away uphill at traffic lights, accelerating at junction, in much the same way we all give it a bit of welly for short periods when we need to but I think that needs to be looked at, not from our perspective but from the perspective of those outside our demographic.
The other question than is the mixing of two almost different forms of transport on mixed use facilities, that's rather more difficult to solve and I see this is a problem in the Netherlands now but of itself I don't think this is a reason not to think about allowing more latitude, like I said, we shouldn't legislate to the lowest common denominator.