I don't know whether Model S has a demand problem or not, but I think you are somewhat contradictory above. If model S buyers are not an entirely different group than 3 buyers, as some people stretched for an S because that was all that is out there, it seems almost certain 3 will/has taken away some S buyers. That doesn't mean there aren't other S buyers who will replace them, but I think is is fairly obvious 3 will take some S demand.
Taking some demand doesn't necessarily cause a demand "problem" which implies demand is flatting or worse, negative. Some of the motivations to buy a model S will be fulfilled by model 3, but the 7 Series or S class customer is not going to buy a 3. So the model S will continue to have similar impact on those brands as they are stale and model S gets better every day it seems (see new 0-60 numbers and million mile drive train). Demand waning for Model S is simply illogical and a problem of math. I made a huge post on demand but I will try to keep light and summary:
1) There are still 19 states in the US that have some restrictions on selling model S, that probably has more impact the model 3. As those issues get resolved, sales will grow in those areas.
2) Expansion of Supercharger and Service Centers for Model 3 will help sell Model S. More of everything helps S/X.
3) Tesla is still expanding worldwide. UAE and S.Korea most recently. These are not tiny markets for EVs.
4) 2014 going forward coming off leases and getting the newest model. The more time the S is in the market place the more existing customers will come back, at about 90% rate, for another Model S. Some of these folks will opt for a Model 3, but if they are a large luxury owner, which most Model S are, they will stick with S. Again, much bigger impact for S in the positive direction then switching to model 3, though we would all be fools to think some wont downgrade. My guess is that the percent is small and those folks originally probably had Prius and Camry level cars.
In my opinion, Model S has probably hit a high point in terms of market share in the US, but that does not mean sales will not grow, because they will continue to get 40% of the market while existing customers come back to purchase their 2nd and 3rd cars. At some point they will hit a plateau for S, but its not this year or next or even the next one after that as the product is just now reaching maturity in the US and has a long ways to go world wide. Model X is still in its infancy as a product and they just basically finally worked through the reservation back log by finally shipping tons of them overseas and the JUST last week.. literally just last week filled the stores with demo/loaner model Xs. I know because 2 weeks ago I had my car in and they didnt have any Model X loaners and yesterday they did and they said they where completely out of loaners until just recently and had been and are still using CPOs as loaners before they got restocked and that they never had model Xs loaners until last week. Model X has at least 2x the potential market vs the S and the biggest issue is not demand, its supply. They cannot build them fast enough.