it is not so.
When you derestrict a hub bike, you will tend to ride faster and as everybody knows, high speed equals higher battery consumption per mile. If you ride on a flat road at 15mph, your motor uses about 200WH and wastes about 50WH in heat per hour. if you ride at 28mph, it'll use more than 800WH per hour and wastes about 200WH per hour.
Your controller will suffer continuously.
The wear and tear of the internal gearbox is also greatly accelerated.
Higher speed equals accelerated wear.
Your brake pads wear faster too. The only bit that wears less is your shoes.
You should feel ashamed in not understanding how hub-motors work. What you're saying would be correct for an unrestricted crank-motor system, but the back emf from a typical 260 rpm hub-motor, like on the Oxygen and some of your bikes prevents that.
With a 15mph cut-off, the motor will cut in and out. The average consumption might be 200w, but when the motor cuts back in, it'll try to accelerate the bike back yup to 15 mph and will take the maximum current allowed by the settings. Under speed control, that would be 15 amps cutting in and out., so you'd be using 600w for 1/3 of the time. Without restriction, the average current would be the same up to 15 mph, but it would be continuous, so the same 200w would be less than 6 amps. After 15 mph, you could use the maximum current to accelerate the bike, but you'd already be going down the ramp where the current gets increasingly reduced by the back emf, until you're only getting about 3 amps at 20 mph.
The Oxygen can't do 28 mph, so that's irrelevant. The previous version maxed out at about 22 mph. IIRC. Even if it could spin to 28mph, the back emf would have reduced the current to zero at that point, so the controller would be handling zero watts, not 800w.