I'll give you my perspective, because I believe I am part of the reason the gap has closed so much between Gen 2s and Gen 3s in regards to the value of each car.
I think a lot has to do with the fact that many guys were still in college, high school, and grade school when the Gen 1s and Gen 2s were still new. Those were the cars guys had as posters on their wall, drove in video games and daydreamed of owning (I was one of them). Prices have gone down since, like nearly every other car has from that time period, and are now affordable for some of those kids who have grown up... So much so that they are willing to pay a premium compared to the newer Gen 3s just so they can own the car they once coveted. These cars can take you back a bit. They bring back memories from when you were younger and the thrill you got when you were lucky enough to just see one drive by. Buying a car that you have always cherished makes you feel proud... Like you accomplished something. Hell, owning a car like a Viper, Ferrari, or Lambo has been a life goal of mine. I grew up wanting a car like that so freaking bad that it motivated me to work harder in school from a very young age onward.
Will the same thing happen for the younger generations growing up around Gen 3s and Gen 4s? I have my doubts. TI believe the main reason is the lack of shock value, as I will mention again below.. Another reason is the youth today aren't as in to cars as they were as recently as 10-20 years ago. There was an article I read a few weeks ago about how the youth today view a car as a utility more than a social status symbol. Kids would rather buy new clothes than put that money into their cars... That kind of stuff. Now I don't really believe this will have much of a noticeable effect, if any, on the kind of people who obsess over cars like Vipers. Guys will always like badass cars.. It's just in our DNA. But as the years go on, more and more people may be choosing to put their money into homes and vacations rather than cars.
One other factor that affects people of all ages is the shock value that came with Gen 1s. Like others have said, Dodge just nailed that design and people couldn't get over the look of those cars... The vast majority of people to this day are still in awe of the Gen 1s and 2s. If you walked up to 100 random people and showed them two pictures, one of a Gen 2 and the other a Gen 3 and asked them to point to the Dodge Viper I'd bet more than 75% of those people would point to the Gen 2, even if they already know both cars are Vipers.
Don't take this as a Gen 3 hating post.. I love ALL Vipers. It's just my opinions on why I think the Gen 2 will hold better value than the Gen 3 from now and into the future.