These synths are a hark back to the early years when only successful musicians with an experimental bend would be able to afford the most spectacular synthesisers, but then the music that would come as from them....
I sorta don't like the idea of artificially limiting the numbers available, but by the same token, an instrument like The River and that CS-80 clone would be wasted on me. Let the likes of Vangelis have them and put some man man beauty into the world with their skill.
I generally agree with that (especially the first half). I doubt, though, that these synths are produced in artificially limited numbers. There's a simple economic reason - not an artificially created one - that makes them limited in the first place: These types of synths are niche, not mass products.
In the market for electronic devices, musical instruments are only a tiny sub group. And within this group you find many specialized niches like analog, digital, or hybrid synths, diverse controllers, etc. And within these niches you again find more sub niches covering everything from beginner's (like the Volcas) to highly exclusive instruments (like the Schmidt), from standardized (keyboard instruments) to more esoteric ones (like the Haken Continuum).
So if you look outside the feedback chambers of synth forums, it becomes obvious what a tiny and unprofitable (by the standards of mass produced electronic devices like computer screens or mobile phones) market this really is, unless your product sells in the region of 100,000 and above like the DX7, the D-50, or the M1. But even that is a rather low number compared to the sales of other electronic devices. Still, if you sell about 3,000 of a product per year, you also can't take advantage of the cost saving options in the production run that apply to really mass produced items.
And then there are also the hardware components for the tactile interaction with an instrument, the field that is not really affected by the technological progress that has made lots of inner components more affordable over the years: A sturdy front plate, knobs, buttons, potentiometers, wood panels, etc.
As for The River: One also has to take into account that this is a project organized by one guy. He doesn't have a large or even a small company to rely upon in terms of resources. In order to not end up in bankruptcy with hundreds of huge synths in his basement, the best thing he could do is to start production based on pre-orders and in test run quantities. If the demand is there and he receives enough positive feedback, he might do another production run, just like Schmidt did. Given the small numbers, any further production and thus the instrument itself won't become any cheaper though.