This is a great point. If they're going to quote miles per gallon as a fuel efficiency figure then they also need to prominently display the battery capacity in gallons. Multiplying the two numbers should give you the range in miles.

At some point it will make sense to change to kW-hours but not while the great majority of cars still run on gas. It's the same with light bulbs where people know how bright a 60W incandescent bulb is but still don't know that that's about 800 lumens. Some LED bulbs are marketed with incandescent-equivalent wattage more prominent that actual wattage.

You are conflating range (time or distance) vs efficiency. First, % battery loss per time would be an unnecessarily complex metric, you would just put the time. That does not tell you how efficient the car is as battery size is not held steady. In terms of a gas car, a Hummer could have a longer time until you need to fill up, but it is obviously not as efficient as a Prius. MPGe while not necessarily the best metric, does provide an efficency metric. It also has a rough correlation to the MPG metric used for years in the US.