Trade-in programs have the effect of artificially increasing the resale value of the product. This is a very conservative move and serves only to reassure people that their investment won't depreciate faster than a known rate. It is most commonly seen in "luxury" goods markets, where status is a significant portion of perceived value AND a significant portion of the original selling price.
A very similar practice is followed by high-end car makers. The styling of high-end cars evolves much more slowly than that of low-end cars. This is to ensure that a new model does not "obsolete" the styling of an older one, driving down resale value.
Let's examine why this is bad for the average consumer who does not want to buy status:
The reason a D100 is worth so little today (practically free) and such a tiny fraction of its original price is that a D100 is technologically obsolete. I think most here can agree on that. The reason we agree on that is because we value a camera's capabilities, not its status. We place a value on that D100 equal to its intrinsic worth as a camera.
If Nikon were to implement a trade-up program it MUST offer the consumer more money for said D100 than the market rate of today. If Nikon does not offer more than the current market rate they will have no takers and the program is effectively nonexistent.
Since logic dictates they offer more than current market rates that means they have artificially raised the value of said example D100 while not increasing its intrinsic worth. Therefore the entire used Nikon market becomes perverted. No longer is one simply paying for capability, one is paying for the promise of artificial value to be refunded, as it were, at the end of a product's life.
Nikon is, like me and I suspect like you, interested in making money. I pay the bills, feed the kids, and hopefully make the world a better place through the power money gives me. Nikon can not burn money over the long term any more than you or I can.
As I said above, Nikon MUST offer more than current market prices for this program to be anything other than a slide on the quarterly Power Point. If they artificially raise the value of a camera the money must come from somewhere, and it won't be them. There ain't no such thing as a free lunch.
I don't know about you, but I want to pay for camera tech. I don't want to pay for status and I don't want to pay for a promise of money in the future.