~400,000 in the first year? That's very optimistic. I know you said "at least", but 400,000 is assuming no more preorders between now and when they start shipping -- how likely is that?
Not very likely, but I suspect that people will start flipping the Model 3 as soon as they're confirmed for the configure and build process.
I strongly believe that the preorder number will do nothing but decrease from here -- that 400,000 are people who have put in deposits -- end of 2017 (or 2018 or 2019) is a long time to wait for a car. I think we can expect realistically some major attrition in that number.
The reason I think 33-35k is going to be the floor for a barebones and low-end used Model S is once the Model 3 is available, The Model S will have been in production for 5 or more years, and have around a total 300,000 produced (currently, the combined deliveries of Model S/X is about 15,000 a quarter -- that's only about 60,000 vehicles a year). Sure, there's about 3 month wait to get your car delivered, but my guess is that they'll continue to build the Model S just long enough to get the factory pumping out deliveries of the Model 3.
If you think about Model X sales to Model S sales since the debut of the X, the S has outsold the X 6:1. There's only a slight difference of around 5k between the X and the S. If we look at Model X -- people waited years for the X, and Tesla ended up delivering all the reservation holders who kept their preorders (2600) within 6 months. For the X, they ended up making just 20 vehicles a day for 6 months to meet the preorders.
The Model 3 is 30k less than the Model S -- and has a preorder/reserved number greater than the numbers of Model S.
If you are a buyer with 70k -- would you rather wait and buy 2 Model 3 70s or have a new 2017/18/19/20 Model S 75/85/95/whatever today?
If you are a buyer with 35k -- would you rather buy a new Model 3 70 or a late 2014/2015 Model S 70 with Autopilot?
Because the Model 3 has such a good range at such a reasonable price, Tesla has essentially created a EV equivalent of a Toyota Camry, Tesla will be selling them as fast as they can make them.
I do believe the Model 3 will cannibalize sales of the Model S, as the Model 3 is 90-100+ % of the car of the Model S for 50% of the price.
Those older pre-autopilot cars will not hold their current value, and I suspect will drop below 33k upon release of the Model 3.