With camera sales overall still contracting, and with so many players duking it out in full frame mirrorless and dangling discounts to grab business, it's easy to see the Dark Side of the situation.
But rest assured, DSLR users, there's a Bright Side, too.
Because so many people who are picking up mirrorless bodies are dumping a fair amount of DSLR gear to pay for their "upgrade" (!?!?!), it's creating a situation in the refurbished, used, gray, and even new market where the prices are coming down for DSLR owners, too.
The thing to pay attention to is the timing of new product introductions.
For example, on the Nikon side, the brand new 24-70mm f/2.8 S is just a far better lens than the old F-mount 24-70mm f/2.8G; heck the fairly new 24-70mm f/4 S is better at equivalent apertures, in my opinion. Enough so that a lot of the transitioners are dumping their big, older mid-range zooms for the newer Z options. Sometime shortly after the Z lens introductions, prices on the oldest 24-70mm f/2.8 F-mount lenses started to drop.
The same thing is going to happen soon with the 70-200mm f/2.8G (non-E) versions, I'd bet, once Nikon delivers the 70-200mm f/2.8 S. And I'm starting to see the same thing with the 14-30mm f/4 versus the 16-35mm f/4. Similar things are about to happen on the Canon side, too, particularly with their upcoming 70-200mm f/2.8L RF versus the older 70-200mm f/2.8L EF.
The tricky part is understanding the timing. This isn't a new phenomena, as we've gone through major transitions like this before (e.g. autofocus, DSLR, etc.). What typically happens is that gear that's considered "out of date" by the early movers starts losing value when the number of transitioners coupled with the better ability of the new gear becomes obvious. Used prices then slide downward for a bit, and then eventually their price either plateaus or rebounds some.
The real issue here is supply versus demand. Lots of trade-ins push supply up far higher than demand, so price goes down. Eventually the trade-in flurry slows, and the supply/demand situation rebalances. You want to buy—and negotiate hard—in the period from the initial oversupply to the eventual plateau.
Thing is, the Nikkor 24-70mm f/2.8G isn't a terrible lens at all. For many years it was essentially state-of-the-art, and you'll find a lot of folk out there still using that lens and enjoying it. It takes fine pictures, but it has some weak points that have been addressed with the newer versions. It's also been in production for 12 years now, and there are hundreds of thousands of these older lenses in the field. So there was already plenty of supply in the used market, thus any new trade-in activity just bumps supply upwards and prices down.
Given that current DSLR lenses on adapters work so well on the Canon RF and Nikon Z mirrorless systems, people are tending to hold onto lenses for which there isn't yet a mirrorless equivalent. Thus, the exotic* telephoto market, for example, is still pretty much performing as it has for awhile (a slow trend downwards in used prices for the more recent exotics, which are overpriced new; a stabilized price for the oldest, particularly the manual focus ones [thus proving my point about plateau]). But watch what happens if we start seeing exotics introduced for the mirrorless cameras: any trade-ins at that point would clearly oversupply the DSLR lens market.
So don't be discouraged. If you've got a D850, you've got the top camera out there, in my view. It's going to perform well and stay viable for many years. That can also be said for even the current top crop sensor cameras (e.g. D7500, D500).
On the Nikon side, D7500, D500, D750, D850, and D5 owners have arguably state-of-the-art gear they should be happy with. On the Canon side, I'd be more limiting, putting the "happy point" really at the 5Dm4 and 1DXm2 (the RP simply undercuts the 6Dm2, and I never really liked the 5DS; meanwhile, the 7Dm2 is very out of date now).
Thus, all those trade-ins from people jumping to mirrorless because that's what social media says they should do are just going to make it cheaper for those of you sticking with DSLRs to pick up back up bodies, additional lenses, and more. Don't worry, be happy now.
* I use the term exotic to describe the very expensive, high performance telephoto lenses, such as (but not limited to) the 200mm f/2, 300mm f/2.8, 400mm f/2.8, 500mm f/4, and 600mm f/4.
