You know, in my not-so-scientific opinion here, I'd say the 9-10K mark is about right on US roads. They are certainly becoming rarer. Here in the midwest, even while they were rare I could count on seeing one SVX at least every other week. Nowadays... the only SVX I see is mine. At least when it's not in the shop
I'd also say it's accurate to say that there are probably 5-6 cars every month that are lost... far more than the number that are actually getting back on the road.
Like the Delorean community, I feel that in about 5 years (15 years after production end) we'll be at about 50% (or about 7500) on the road... and there it'll stabilize. Now, when I say "on the road" I actually mean "able to be driven"... not necessarily "driven every day". That number will be a lot lower... maybe 30%... maybe less depending on how well SVXi survive the next couple of years.
I have a friend who's a real big DMC fan... he has two. I've driven one of them, and they're sweet cars (though extremely dated). He reckons that today... some 24 years after the last model year of production there are maybe less than 30% roadworthy... with less than 10% being frequently driven. However, they have become more collectible in recent years, so their value is in fact rising.
Is this what the future of the SVX will hold? Maybe, maybe not. The number produced and the similarity of the fanbase (in terms of income and personality types... the only place they really differ is in age) lead me to believe that the SVX is following an extremely similar pattern. The DMC gained much more value much more quickly due to "star value" (ref: BTTF) but still their value really spiked shortly after the release of the second movie and stayed relatively stagnant since when considering inflation. SVX never had this "boost", but I see the numbers of driveable cars following extremely similar numbers. In fact, the SVX numbers may be lower because they don't have the "coolness factor" provided by Hollywood.
In short? The SVX probably won't be a collectors car. In a few years we'll hit 50% of the total number imported actually roadworthy. At that point, the fans and collectors will actually represent the majority of owners (as happened with the DMC), and then the attrition will decrease. However, as much as we might want to the cars won't run forever... and neither will parts be available forever.