I kinda know what you are saying but equating what the musclecar market has done in the last 10 years to where the mk3 is headed is a bit of a stretch. The one thing the mk3 has going for it is relatively low initial interest when it was sold. As more and more hit the scrapyards or get parted out the price is certainly going to appreciate for good examples, but will Supras routinely go for 30-50k in 10 years (putting them at about the same age as the current 60-70's musclecars are now in terms of age), I would be willing to bet against that. This is, and always will be a country where American Muscle will command a higher resale than Japanese "Muscle".
Supply and demand requires both sides of the equation need to be at the peak for the value to be driven up. Supply is certainly dwindling, but the demand will NEVER equal that of the american muscle car. Of course, thats just my opinion and I have no facts to back that up. Only time will tell.
Given the choice right now of buying...say a 70 Nova (not exactly a high demand car then nor right now for that matter) and a mk3 strictly as an "investment car", well thats an easy call. Im not dissin' the mk3 mind you, but a smart investment it is NOT. Hell even a Honda is a better investment car simply because demand is still rediculously high. Again, dont get me wrong, would I rather drive a mk3? sure!