I don't even think they have to be cheaper than current cars, they just need to get cheaper than they are, get much better range and much faster and more convenient recharge than today's electrics get.
The top of the line Model S gets around 315 miles to a charge (X's get a bit less), it was a world changer because it was an electric car that got about what a normal ICE car gets in range so there was little penalty there. Not many people need to directly drive from NYC to Toronto in a single stretch all that often.
bonus: the cars are stupid fast and have incredible amounts of storage so they're better than other cars on the market
negative: they're very expensive and recharge still takes lots of time or less time but with expensive home infrastructure (but still not gas refill time)
What makes them exciting is that for a couple of downsides (price and recharge time) you get a car that's at least as good than old fashioned ICE cars and better than them in most other areas. They represent the least compromised vision of electrics you can buy today.
The smaller electrics aren't getting near that yet...it will take a few years of battery improvements before that happens. I think lots of people want a Model 3 aspirationally hoping that it has some of the positive qualities of an S, but the reality is that they're much more compromised vehicles. Tesla's approach to dealing with this is to try to make the cars as incomparable to ICE's as possible with loads of electronic features like autoparking and autopilot and whatever and an interior that's not like anything else on the planet -- but they're the only electric car maker who really gets that. By making the Model 3 hard to compare to anything else, it makes it stand out.