Model 3 and Model S are different cars, sure. But that doesn't make the process of building them different. How many more parts should S have to go about its business? How much more aluminium and steel to make up the chassis? Seats? Wheels? The variances are minimal.
With Model S having such a ginormous marginal profit, already before the 2170 cells which are bound to cut a huge chunk alone, the door is open for external competition. The time where other brands weren't interested to make a premium BEV for the masses (2012-?) is long gone. A $70K or so Model S will be a hard sell when Volkswagen has an electric Passat, Polestar a Volvo placeholder, etc, etc, at a lower price point.
Why are there counterfeit handbags on the market? The originals are so highly priced, it becomes worth faking/competing. Even when it's clearly an inferior product, a lower price does wonders.
Back to topic. What might the practical advance be to put the Model 3 (small?) rear motor inside the wheel base, how will it affect trunk space? Will the trunk go really low?
Model 3 lacks the stacked modules in the front. What's do with that space, of the frunk is so much smaller?
The ongoing 18650 ncessarily contract overlapping Model 3 production, may have influenced them to make the Model 3 pack on the small side. Model S, if the 2170 cells are so awesome, should already pack 125kWh the moment it's converted. They could eek out 100kWh a bit on the tardy side in 18650 product life, to keep Model 3 reasonably big and not threatening Model S. If not for sale, then for image.