What do you mean by "very limited geographic availability"? You mean in the US, or worldwide?
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I do think sales for MS/MS will be leveling out in the upcoming year. Good thing Tesla has the Model 3, Model Y, Pickup truck, Semi, and Roadster in the pipeline (and who knows what else, motorcycles?). The company can no longer survive on MS/MX alone, which has been their bread and butter up until now.
I mean both US and worldwide.
I think you're correct about leveling out of MS/MX until they do a model revision. Technology has advanced a long way since the 2010 basic design of the drivetrain for MS and MX. IT is time for a serious update, which should produce longer range, greater efficiency, less passive drain and, crucially, reduced manufacturing complexity, lower parts counts and cheaper sourcing. In the Tesla BEV world that makes the MS/MX obsolescent technically. I do think there will be a serious revision (probably not 'all new' certainly not a 'refresh') within the next few months, prior to Model Y, just about when Model 3 hits 5-7,000 per week reliably.
Among other things that will allow adoption of new motors, vastly reduced wiring, 2170's, interior updates and redesign to permit increased use of robotics (e.g. interior assembly).
Once they do that MS and MX will again be capacity constrained even by your definition and production costs will be reduced.
It will be much less expensive for them to do that than it may appear because they already have done the R&D VAG-style, thus allowing effectively modular adoption across a variety of dissimilar models. After all that is one key reason why Peter and Fritz are there, is it not?