I don't have sales figures for the years the DI was offered in the US but knowing how small the dealership infrastructure was (owning new Rovers back in the '90's) I know that sales numbers from that period are not greater than those being reported these days. Rover offers many more vehicles and has significantly more models than they did in 1996 (probably the biggest selling year for DI's). In 2011, Rover, offering the Range Rover, Range Rover Sport, LR4 and LR2 sold 38,099 units through well over 200 dealerships nationwide. Back in 1996 Rover had fewer than 100 dealers selling just the Range Rover and Discovery (Defender numbers were even more insignificant).
So, the principle reason you're seeing fewer and fewer DI's for sale is that of the small number that were actually sold new, fewer are still on the road in condition for resale. Off the top of my head, maybe Rover sold 20,000 DI's eighteen years ago across these 50 states. Based on normal attrition pick any 18 year old vehicle and ask yourself how many you see for sale. Then think about how many were sold new compared to the paltry numbers Rover puts up.
Now, as far as having DI's turn up in junkyards, I'll admit that you do see them on occasion. However, their numbers are far more rare than comparable SUV's from that period. You'll always see an array of Jeeps, Nissans, Toyotas, ect. but I've never seen more than a few of Rovers in the same yard. Many in my area have never had a Rovers in their yard. That's saying something considering I live about 80 miles north of NYC, minutes outside of Fairfield County, CT (one of the richest counties in America). Even though I am in an exceptionally rich Rover area, the numbers still are nothing compared to other brands.
Finally, regarding value, I think we are a very long way away from DI's achieve collectible status. The Range Rover Classic is a different beast. It was sold in absurdly low numbers. The best year (and extended one at that) was the 1995 MY that started in 4/94 and continued well into 1996 when supplies finally ran out. I believe 5,600 Classics were sold that year. That's well above the second best year which was around 4,500. There were years were Rover sold under 2,000 Classics making a grand total of just over 35,000 units in nine years! I'd estimate that four or fives times that many of the cheaper, less refined DI were sold.
It's really only been in the last five years that I have seen a massive turnaround in the Range Rover Classic market. In the early part of the last decade their value had fallen to the point where early models were being snatched up as offroad beaters. I know because I bought plenty for parts and they seldom cost me more than $500 a piece. Most of those poor souls wound up parted and crushed. Lately, there is a new dynamic in the Classic market. I'm seeing people paying well into the teens for a quality, rust free Classic and they are willing to spend real money on quality replacement parts. Later LWB's and in particular the '95 soft dash are leading the way in establishing new price points for vehicles and parts. They are actually on their way to becoming classics in more than name only.
On the other hand, I feel that the DI is much farther away from moving from offroad beater into the realm of collectible. They never had the panache of the Range Rover Classic or the sales numbers that add to the exclusivity. Honestly, at best, I see DI's recovering a bit in their value over the next ten years and yes, you will see the near perfect survivor go for a lot more money than expected but that will be the exception rather than the rule. In the end, I doubt I will see either the Classic or a DI ever approach a level of collectibility that a genuine Dodge Superbee has. They'll more likely conform to the way early British Leyland TR6's and MG's have appreciated, still well within the range of any real enthusiast.