This is exactly why I expect it not to happen. When cars had 100 mile ranges, people said they needed a 200 mile range (or whatever). Now with a car with a near-300 mile range, people will say 500 (or whatever). Then if it's 500, people will say 1,000. And so on. The numbers people throw out are completely arbitrary. People simply don't know how much they drive, and the ones that think this way won't get the car until EVs become normal to them. Which will happen as the rest of us drive them and show the world how much better they are and how they work despite not having the things they think they need.
A lot of people also think their gas cars go 600 miles between fillups or something, but compared to cars in it's class, the Model S is right smack-dab in the middle of them all in terms of how far it can go on a single charge. Or that they routinely drive over 300 miles a stretch without stopping, and last time this conversation came up here, everyone responded saying they did it once when they were young and will never do it again.
Meanwhile, huge batteries are larger, more expensive, make the car slower, handle worse, less safe, brake worse, less efficient, etc. etc. etc. Manufacturers want to keep costs down and want to make cars better, and all of these things need to be compromised in order to add more range to a car. Manufacturers have looked at data of how people use their cars, and particularly Tesla has a lot of this data. They will know that a vanishingly tiny percentage of the population would ever use any more than the current range of the Model S in a day, and they will target the 99.9% of drivers rather than the .1% of drivers because it would be silly to focus on a niche when you are trying to make a mass-market car.
The focus will be on supercharging, and possibly on giving people some sort of buffer for weather (though Bjorn did what, 250 miles in heavy snow and mountains?) through some kind of technological improvement which may or may not be a larger battery (I could see them going with a larger one to make up for higher weight on the X, then later migrating it into the S, because why not), but we will not see ever-increasing ranges on mass market cars - the same way we don't see ever-increasing gas tank sizes.