This is just my observation from being on this rock for 52 years, I could be wrong but here goes. I think like anything else automotive related certain vehicles of all brands strike a chord with the buying public when brand new for whatever reason be it looks, utility, horsepower, whatever. Muscle cars in the 1960s and early 1970s before EPA emissions standards kicked in is a prime example.
So when these vehicles are new they sold in large quantities. People bought them and used them for their intended purpose as transportation, or in the case of pickup trucks for work use. When this stuff is 5-7 years old and the warranty is gone or close to it most owners would do a trade in.
It's just a used vehicle now. New car smell and excitement is gone. A lot of these get used and abused harder as older vehicles because the investment in it as a used vehicle is substantially less. Inexperienced younger drivers buy used for their first cars and tend to have wrecks. At about 10-12 years old, vehicles in the rust belt states are starting to show corrosion issues in the rockers, fenders, and floorboards. Junking a low mileage vehicle with severe rust issues up there is quite common.
By the time the remainder of the vehicles hit the 20-25 year mark, theres a large attrition rate. theres several examples of what's left. The first being the Low mileage (creme puff) ones that have led pampered lives still wear original paint and are generally well cared for. These are called "survivors". In the muscle car world these ARE the holy grail since muscle cars had a tendency to be modified from day 1 and driven into the ground so to speak, plus high rates of wrecks with street racing.
The second example is the nice daily driver. It's got some dings, scratches, paint fade and miles. It's been used like a well worn in pair of jeans. Not really abused but not fanatical care either. These are easier to find and most of what you see running about.
3rd is the poor condition one. It runs, but has been neglected to the point that it's a parts vehicle unless you got some emotional attachment to it. Can do most of the work yourself, or have deep pockets to fix it up.
This is just my observations with dealing with this old stuff. I remember when 15 year old beat to **** muscle cars were $500 cars running with a title. I almost bought a 1970 383 4 speed cuda convertible in 1989 for $1600. Those days sadly are long gone. I walked away because of the rust issues. AMD where were you then lol.
In my long winded way of saying this, these GMT400s just like anything else automotive, goes thru phases in its lifespan from new, to used, to junk, and once they arent seen in large numbers on the road, all of a sudden people find them again and say "hey" this a good looking vintage vehicle. I want one. Or the person who had one new, and decided they want another one, maybe they regretted selling the other one they had.
Back in the early 90s my parts getter was a 1971 C/10 custom. Good truck, it was what I wanna call a #2 condition truck. I swapped the 6 popper for a mild 350, and a muncie 4 speed. Doors were rotting on the bottom. And the cab was heading that direction too, ditto for the radiator support. I lived in N.J. at the time no rust free steel there, internet was not up and running. No real way to network to get rust free steel from somewhere else to repair it. I made it look decent, and sold it to buy my 94 silverado that I still have. Now I find out that you can now buy new cabs, beds, doors, tailgates, and everything else to rebuild one of these. At the time I had mine that replacement stuff did not exist, since this truck was built in large numbers between 1968-1972 and there was a resurgence in people wanting these again, the aftermarket responded with parts.
GMT400s were built in record breaking numbers from 1988 thru the 1990s. The reason for that was Dodge still had what was essentially a 1972 cab and frame design with several facelifts. The only thing that sold well from 1990 up was the 3/4 ,and 1 ton trucks with the Cummins diesel for farm and industrial use. The ford trucks were also an old dated design. The GMT400s however were revolutionary at the time. As an all mopar guy back then, I was completely wowed by this truck when it came out. GM finally got something really really good to replace the awful 73-87 rot boxes they built. They fixed the corrosion issues in those in the early 80s but those were still junk in my opinion. I live in Texas now, dry climate hardly any rust and I see people here fixing up the 73-87 C/10s. Being as I lived up north and saw these things fall apart in 5-7 years I still call em the rot box years. That's how bad of an impression they made on me.
GMT400s you dont see many on the roads anymore even here in west Texas. A lot of em in the salvage yards now though. Like worn out 25-30 year old muscle cars started to be restored and rose in value, the same is happening for these trucks. When I got back into muscle cars again back in 2008 I wanted another 68 dodge charger like I had as a teenager. I was shocked to find that prices were $3500 or more for what essentially was somebody else's stripped out parts car with a vin and a clear title. This thing was a gutted unibody with axles and a roof, and little else. A not quite complete basketcase. I hope it doesnt get that way with GMT400s that a numbers matching frame and stripped cab with clear title costing a couple grand. This may or may not happen when these things reach 50 years. It all depends on the attrition rate and what's left.
I am still amazed at the muscle cars people are still finding in garages under inches of dirt, or in barns, back yards etc. I know this isnt a clear cut answer except that as these get older and less clean examples are out there, it will start to drive up the values on the decent ones that are left, where that number goes is anybodies guess. These will probably be valued more in the rust belt states where clean rust free examples are all but non existant. These will become the garage/show queens that get put away 6 months out of the year because of the winter road salt and its effects.
When those clean rust free examples become harder to find, then the basket cases and picked over parts trucks will start to get hauled out of fields and backyards to be rebuilt. The aftermarket will gear up to make even more replacement pieces than is out there now because there is interest in these. I am now just waiting for exact duplicate 60/40 bench seat silverado light grey cloth repop covers to come out, since mine are starting to show some wear at 26 years. I want exact duplicate of OEM. Theres nothing yet.
Hope this helps
Matt