The issue you're seeing isn't the truck falling out of favor at all. It's 2 things:
1) A lot of people have this dream - from their cubicle in the Matrix - of this wild, adventurous lifestyle of exploration. But the reality is that they put together the perfect vehicle for it, and then have zero guts and nuts to actually leave the Matrix and do said exploring. So then their spouse and their accountant convince them to sell it, lol.
2) The demographic who goes for the 5th gen, ie, middle management Matrix guy, also has a severe OCD prepper/consumer-whorist mentality, as detailed in Fight Club with the Ikea catalog. So there are a ton of folks who are "building" and "instagramming" these brutal battle-beasts that are stock weight (4,500 lbs) +1,500 lbs of steel armor, wheels, bumpers, etc, in pursuit of the perfect stance and look... and then they realize that all the race-ready and aftermarket parts aren't OEM R&Ded, and thus require a metric sh*tton of maintenance and bi-annual rebuilds and lots of grease to keep away the squeaks... and they end up cutting their losses and selling.
So that's what you're seeing.
That said, I do have an uncanny ability to see the future, and for about a year now, I've been wondering how this forum's for-sale section will look when gas hits $5/gallon. I think there will be some tasty part and used built-trucks for sale lol