By design, there is always a reserve in the traction battery. A reverse gear engine backup would then only be needed if you needed to go a couple miles in reverse. 50 feet of reverse covers perhaps 99% of back ups.
Think of how great a transmission we would have if the motor was exclusive to the first 6 mph! The motor would be first gear and reverse, and now we would have effectively a seven speed.
2018 Kia LX HEV Metal Stream with Advanced Tech