The reality is that most people don't drive more than the Volt's 38-mile electric range most of the time. Obviously, individuals have different habits so there is no one true answer here -- but most people have a commute between 10 and 40 miles. If you go to voltstats.net, which is a Web site collecting driver data for Volt cars, you can see the driver statistics. The fleet median is 79% electric driving. There are numerous people driving over 99% of their Volt miles on wall-power electric.
My point here is simple: For many people, the Chevrolet Volt is a 90% electric car, a 99% electric car -- or on average a 79% electric car. It drives like a 100% electric car for those 90% or whatever miles -- not like a hybrid, unlike many other plug-in hybrids. Many people drive for up to six weeks before the Volt's gasoline generator turning on.
So yes, Tesla is -- and will be -- a 100% electric car. But really, do most people care? My sense is not. For most people, the Volt's 90% ability is good enough, especially if it comes with a much lower price tag.
There are of course a variety of puts and takes in this comparison. Let's take a 200-mile all-electric car such as the third-generation Tesla in 2017: 200 miles is great, but you won't get 200 miles on a day when the temperature is below the freezing point. Also, after a few years you will lose perhaps 20% of the battery capacity.
A 20% reduction in battery capacity now means you have 160 miles of range. Then wake up on a cold morning and you could be looking at 130 miles of range. That may be enough for many people, but it's not 200 miles anymore.
Of course, when the car is new and you are driving in flawless California climate at an even speed of 50 mph, you will likely get a lot better range -- perhaps even 250-275 miles. So 200 miles is only an average, and when the car is new.
In contrast, a Chevrolet Volt will not see much impact to its 380 miles of range -- whether on a cold day or when the car is old. The Volt could lose its entire 38 mile electric range (I can see half as being realistic on a cold day a decade into the future), and it hardly makes a dent to the car's usability for longer trips.
The puts and takes between all-electric and the Volt extended range model continue. An all-electric car is liberated from all sorts of heavy and complex equipment taking up space: gasoline engine, transmission, fuel tank, exhaust system and so forth. As a result, you free up more space for people and their luggage. You could also get better handling.
On the other hand, an all-electric car needs a much bigger battery. That's not something that scales infinitely. A bigger battery means more weight, which in turns requires a bigger battery, which in turn means more weight. You get the point.