I don't think that small 38hp range extender could power the car for much farther, it's not powerful enough to run the car indefinitely at highway speeds (there's a lawsuit against BMW for the car slowing to 45mph while using the range extender), it needs some battery capacity to fully power the car.

They could put in a more powerful motor than that small scooter motor, but then that costs and weighs more, and then the car becomes a plug-in hybrid rather than a battery electric car with range extender.

And there seems to be low demand for plug-in hybrids as there aren't many options.

When I bought my Honda Accord Hybrid, I looked at the Clarity plug-in hybrid but decided against it. It didn't drive as nice as the Accord, cost nearly $5000 more, plus I was doing some regular long distance travel at the time, the accord has a ~500 mile range, the clarity has a ~340 gas+electric range, though the plug-in range would cover my commute.

And the Clarity was rated 40mpg highway, while I get around 50mpg @ 60-65mph with the Accord Hybrid (drops down to 44mpg at 70-75mph)

From what I read looking into the situation, it is barely enough to run the car at highway speeds in general. The lawsuit is mostly because the US version of the software won't turn the range extender on until the battery is almost completely empty. If you could turn it on at 50% battery you'd have almost no problems.