I think about it exactly backwards from that. The new trucks cost so much that the used prices have plumped because the demand for something cheaper is so high.
The fact is that trucks are bigger, more powerful, have more electronics, more convenience features, more standard features (like HID headlights) etc., that keep driving the cost up.
Can you really put a price on the ability to update Facebook while towing?
If a 10-15 year old truck were put back into production today, it would be cheaper than the current models (adjusted for cost of materials and inflation obviously).
Honestly, this was what I faced when looking for a smaller truck in 2015. Frontiers and Tacomas had been in production for about 10 years with the same generation, and held their value really well. It wasn't much more expensive to buy a new one than a good condition used one, and I wasn't about to buy a dated, 10 year-old design as a new vehicle. So I bought a new Colorado.