You don't have to ride it at @ 40mph you know.
Just like I don't have to drive my Cosworth at 150mph
But there are times when I've had it @ 140mph in a safe and controlled manner
Slightly off topic but the escort cosworth was my dream car when i was younger and still like to see them on the road. But i came across one on my kawasaki ZZR 1400 and the guy was just a jerk, to such an extent it was in my opinion dangerous.
Talk about boyhood dreams being shattered, taking away from the lights the guy gave it every thing but as this point i had had enough. I really was surprised at just how slow it was, it was a dot in the mirror after 4 seconds and that was the last i seen of it.
I would not even say i gunning the bike as the road was damp and i had it on low power with traction control on, but it still looked nice. I should add that not many cars can keep up with that bike so the poor old cosworth probably never done too bad just things have moved on but it was a lovely example just a jerk behind the wheel.
Back on topic, the won't use the throttle argument does not wash with me. When i bought that bike i was determined i would keep the throttle back and treat it with respect, 95% of the time you do. But human emotions come into play like that cosworth jackass and i used it pretty hard. And the times when you do use it are most likely when you are angry so not the best.
You get used to extra speed it's amazing how quickly, the problem with the zzr is how easy it does it, even exotic cars ares disposed of with ease. So my point is don't kid yourself saying I won't use it, you will as 25 will start to feel slow then 35 and so on. Trust me I used to think 100 mph was fast until you have a machine that can reach it in 5 seconds and it's amazing how quickly you get used to this, but at least the bike has the stopping power and technology behind it unlike the DIY job on eBay.
I really would not like to hit a pothole at 40 on a cheap push bike you really are in the realms of needing proper suspension and frames designed to go that speed. Now if they said 25, then probaly ok for a bike frame, remember the base bike is cheap designed for 15 mph or thereabouts but whack a heavy battery on and this stresses the frame more.