Replying to the thread title, I believe that launch of the Model 3 will absolutely (and finally) help us understand the true demand level for Model S / X. A confounding factor to understanding the demand today are the large number of people that are buying a more expensive car than they are comfortable with, or have ever purchased before, when they acquire Model S. These are people that can afford the car, but were never in that market before, and given a choice, wouldn't be in that market now.
I'm one of them, and I'm pretty sure that I'd take a well optioned Model 3 for $50-70k over S or X in a heartbeat. Particularly in more of a station wagon type configuration
Anyway - back to my point. The lack of a full function / compelling EV in the ~$50k range is driving buyers in 2 directions. A small number are going upmarket and buying an S/X anyway - a larger number are going down market for something to make do with, or even further down market and doing nothing (no EV at all).
When the next major price range starts having full function / compelling EVs available, then we'll start to see what the true demand is for a ~$100k EV. People that would rather spend $50k than $100k and who can get their needs met with the $50k vehicle will do so, and to that extent, S/X demand will subside by that amount. Of course we won't know immediately upon availability - it'll take a year or three and enough supply in the $50k and then $35k range to know what the true demand is in the $100k range. In the meantime, Tesla should absolutely take advantage of some people being able and willing to go further up market than they really want to go, and make and sell as many S/X as they can. By my reckoning, they have about 2 years ('16 and '17) to really make hay. After that, it'll be real demand - not the artificially created demand from no options we have today.