Well if by that time Tesla is still only capable of doing one model at a time then are doing something wrong. They should be able to do a truck and another sedan.
On savings I disagree on the relevance. People tend to mostly look at the price at purchase. And many would find it hard to spend $35K on any car.
Perhaps the model 3 will simply have longer range over time. But that raises the question - is it a given that the 8% rate improvement in li-ion batteries continues that far?
Ann Marie Sastry said in a presentation that it is plateauing already. With any such rule eventually you hit physical limits.
You can't treat Tesla like every other car manufacturer out there.
I fully expect the Model S to still be in production in 15, 20 years. That Model S might be sporting a 150 KWh battery, have parking lot to parking lot autonomy, have several more external aesthetic options, be made out of more advanced materials, and have a 2.3s 0-60 performance option, but it'll still be a Model S at its core.
They're supporting hardware upgrades to existing Roadsters, a car that never was truly mass produced and has been out of production for many, many years.
People need to fully think of Tesla as a software/technology company, bringing the 100 year old automobile into the current modern age. You don't see Apple abandoning the iPhone nearly a decade after production--it still looks fairly similar to the original, but it's undergone a huge number of iterations. Same with the iPad, Apple Watch, and their personal macbook lineup. Tesla has abolished the concept of a model year. Improvements come fairly quickly and in smaller batches, rather than in discrete model years. Older models still receive support via OTA (much like older smartphones) while newer iterations have an increasingly vast number of hardware improvements.
I imagine Tesla will eventually have a base model for all the current popular archetypes (sport coupe, microcar, truck, full-blown SUV, smaller and larger versions of each, etc.). You can bet that Tesla will be slowly evolving all of their vehicles year after year, but I highly doubt you'll see them one day discontinue the Model S, and replace it with a similarly-sized but wildly different family sedan like you see with manufacturers now.
The similarities are striking between smartphones and Tesla. They're expensive and novel products, but people who previously never paid $650 for a phone are doing so, and are replacing that purchase frequently. They go through small but important iterations, both through hardware and software. Some people must always have the latest version (P85D folk who used to have P85+), while some will still be rocking an S40 for a decade without an issue.
I think a lot of people, even those in the pro Tesla crowd, are not prepared for an equally transformative effect on our society within a short period of time that will happen due to Tesla. It's not just about EVs, it's about how the concept of an automobile is going to barely be recognizable in a decade, and current Tesla owners are getting the first taste of what's to come. I wouldn't be surprised if, just like the smartphone revolution, we see only a small few manufacturers adapt fast enough while the rest become as unimportant as Blackberry.