I see this one all the time: “I lost money when I bought my X for US$Y and it subsequently depreciated in value.”
Businesses don’t (or shouldn’t) have this same problem. The calculation for a business tends to be simple: does the value provided by an item exceed its depreciation?
Consumers, on the other hand, seem to struggle with the fact that most things they buy depreciate in value and that they probably shouldn’t be buying things that depreciate more than the value they get out of them.
Cameras and lenses don’t appreciate in value. Well, at least most of them don’t. There have been a few exceptions over the years. But you shouldn’t count on your choice of camera/lens being one of them.
From first sale to closeout sale, cameras tend to drop about 25% in the price you'll pay for them. In other words, if the latest/greatest announced camera is US$2000 at introduction, by the time the last units are sold new, you should expect to find them at US$1500. Lenses have longer life cycles, so they tend to sell at list price at introduction, then get periodic ~10% off sales to occasionally clear built-up inventory (or sustain a production rate).
What a used camera or lens will sell for has been in a bit of an adjustment with the transition from DSLR to mirrorless. A DSLR camera or lens just doesn’t have the same value it used to. And a dealer is only going to, at best, give you 50% of what you could sell it for yourself (they need a profit margin in committing funds to it and later reselling it). We have a lot of originally US$3000 DSLRs that are selling used for anywhere from US$500 to US$1500 these days, and that’s going to continue to slide downward as the seven-year manufacturer guaranteed repair cycles expire. Take 50% off those numbers if you're going to trade it in, and, well, your older camera isn't worth much, is it? (Tip: sometimes camera makers will put a bonus on a trade-in via your dealer. Ignore those bonuses at your peril.)
Most consumers I hear from are paying for state-of-the-art product and then complaining when it doesn’t hold value. A simple solution to that is to buy at the back of the trend, not at the front of it. With new cameras near end-of-life, that means a 25% discount for waiting. With new lenses, it means simply waiting until it comes on sale.
Of course there’s a gotcha in all this: what’s the value of being able to take photos with a new product today? A good case in point is the Nikon Z9 and its Auto capture capability. If that function nets you photos that you couldn’t easily get otherwise, you have to assign a value to that. That’s particularly true for us sports photographers, where we’ve long been trying to use multi camera setups with remote triggers. If we get the image that the others at an event don’t, that has a huge value for us.
But we’re back to businesses—sports photographers in this instance—who can put a direct value on something. Consumers using Auto capture would have what? A ego burst over the poor enthusiast that’s trying to get that same image manually?
Which brings us to the real culprit here: marketing has made it imperative that you keep up with the Joneses. If Jonesie has a Z9, you need one to feel like you're still in the same league. And yet, I'm constantly seeing a stream of photos from older cameras that really hold their own and tell unique stories.
Here's what I'd suggest you compare instead of the capability of your camera or the size of your lens: the photos you actually take. Are they as good as they could be from the gear you have? No? Then you don't need new gear, you need training and practice. Both of which tend to be less expensive than buying new cameras and lenses.
Most people don't like hearing that, as they don't like hearing that they're at fault. It's easier to blame it on the gear.
In my long career in photography I never felt I was as good as my mentors and instructors. But I also never felt it was that Galen Rowell used better equipment than me (he didn't, he used two older, less expensive lenses than I had). What I tried to learn from him and others was how best to use what I had. I didn't consider it a failure that I wasn't as good as him, I considered it an aspiration to get as good as him.
I'm as much as a gearaholic as any who have GAS (gear acquisition syndrome). I'm just curious about new stuff and what it might do. But I try not to let that play into my creations. I was (almost) as happy running around Botswana with the Z8, 70-180mm 2.8, and 180-600mm f/5.6-6.3 VR lenses last month as I was in April with the Z9, 35-150mm f/2-2.8, and 400mm f/2.8 TC VR S lenses. In fact, I was happier in one fashion: my more consumer gear was easier to carry, stow, and protect. (I wrote "almost" because f/2.8 at 400mm versus f/6 does make a difference to backgrounds, and I'm spending a lot more time these days concentrating on things behind the animals.)
Value what you do with your gear, not your gear itself.