How much range you need verses how much you (think you) want can easily be quite different. BMW chose the range on the i3 for several reasons:
- the majority of people commute in the order of 34-miles RT on a daily basis...close to double that gives a decent buffer on the original one
- while battery energy capacity is getting denser, larger capacity adds both to the costs and weight and the required size to hold everything which dictates other things...the process is a balancing act - get better with one factor, you lose on another. The last battery update was heavier, but the same volume...the next one may need more volume as well, affecting the weight, size, and overall efficiency. Increasing weight tends to decrease efficiency...it takes more energy to start to move the vehicle and to climb grades, it can also increase drag as you then need larger tires to hold everything up (one reason the i3s is less efficient is because of the tires on it).
Many people that bought their i3 with the REx, have only used it to test that it works over years of ownership. AT least a few have replaced their Rex with a BEV when their lease ran out because they now realize they didn't really need it. There's a learning curve to feel comfortable with new tech, and the dealerships are not really very good at explaining, and the commission on the higher cost REx version probably deters them from pushing the BEV as much. Knowledge is king.
When the next generation of batteries make it to full scale production, higher range will be both more economical, and still not require major increases in volume or weight. It may be another 3-4 years before we see a major improvement, and prior to that, expect incremental gains.