I really do think that demand for Model S and X is limited basically by who has enough money to buy 'em (and who is willing to spend that much on a car). Tesla's approaching total control of this price bracket (segment) -- if you compare it to the other cars in the price bracket, Model S is heading to over 50% of the market IIRC -- and I don't see the number of people willing to pay that much for a car expanding much.
To on develop this thinking, let's use an Audi example:
The lack of availability of other long-range EVs has already pushed the Model S into segments of buyers that would normally never consider something in the Audi A7/A8 range, which is the Model S group. We have tons of people coming from a Prius to Model S on this forum, for example. That is not the story on a forum about Audi A7/A8, definitely not. This is why the likes of Model 3, I expect, will hit Model S harder than a new Audi A4 hits the sales of Audi A7/A8...
Beyond that, I would argue the split between Model S and Model X could be more nuanced. Let's look at Porsche:
My basic proposition is that Model X is inherently in a much larger volume segment of cars (high-end SUV) than Model S is (high-end sedan/hatch). In 2015 Porsche delivered (
Porsche Geschäftsbericht) 17,207 units of Porsche Panamera, a high-end sedan/hatch like Model S. In the same year Porsche delivered over
four times as many Porsche Cayennes at 73,119, a high-end SUV similar to Model X. (And again, Audi also selling tons of Audi Q7.)
Now, we can argue the practicalities. Porsche Panamera as a four-seater is a somewhat compromised hatch, whereas Cayenne is a fairly high-utility SUV (fully folding rear, roof rack expansion). Model S is a high-utility sedan/hatch with up to seven seats and massive storage thanks to large trunk and frunk, plus the roof rack. Model X, in reverse, is a somewhat compromised SUV with large area folding seats only recently and only in the fiver (no edge with seven seats) and the unconventional doors nixing the possibility of using roof racks and, I assume, alienating some more conservative buyers...
So, it is possible the relative lack of utility in Model X can limit its market appeal more, and likewise the relative abundance of utility in Model S can expand its market appeal more, but still at some point they both come down to what they are: an expensive sedan/hatch and an expensive SUV. Electric or not, those categories do have certain ceilings overall and relative to one another, and the expensive SUV category in all likelihood is larger than the expensive sedan/hatch category is...
Especially when things normalize competition and availability-wise, i.e. when there are more alternatives, I fully expect Model S to become the lowest selling Tesla instead of the biggest selling one of the S3XY lineup - unless the falcon wings prove to be a fundamentally problematic proposition on the market once production constraints on the Model X clear.