Lately my equipment doesn’t become outdated as quickly as it used to. Part of that is the technological maturity of digital cameras, but my maturity as a photographer also has an effect. I shoot with a Leica manual focus digital rangefinder camera, whose basic design dates back to the 1950’s. It has a very simple center weighted exposure meter that I use most of the time, and I bought the amazing Lumu exposure meter attachment for my iPhone for times when I want to meter externally. (There is also a matrix metering system in the camera, but it’s not as fast or convenient to use, for various reasons.)
Because I don’t rely heavily on leading edge technical features that are undergoing rapid change, my camera is still completely satisfactory almost five years after it was originally announced, even though Leica has released two new M-series cameras since I bought it. Only one of those two has a new sensor, and its main advantage is about 1.5 more stops of high ISO performance. My sensor resolution is already 24MP, more than enough to make large prints. (I’ve learned to compose in the viewfinder, so I seldom need to crop much.) I transfer my photos to my computer via a card reader, so WiFi (available in the two later models) doesn’t matter a lot to me.
There was a time when sensors were improving rapidly and substantially, and after that we saw significant improvements in autofocus technology and performance in mirrorless bodies. Those improvements have slowed down. Improvements now will be largely in the area of processing, as techniques used in, e.g., the iPhone XS and the Light camera start making their way into cameras with larger sensors. If you buy an excellent body like a Sony A7R III or a Fuji X-T3, it’s hard to see why you would want to trade it in for the next model any time soon, unless you want 8K video or 50MP resolution. (The Nikon Z and Canon R are a slightly different case, since they are the first of their line and should be expected to improve more rapidly in their next iterations.)
Leica digital rangefinder with lens from the 1950s.
I should also point out that concerns about obsolescence are even less relevant for lenses. I can and do mount Leica lenses designed in the 1930s and manufactured in the 1950s on my 2015 M-P Typ 240. My Leica Summilux 50MM f/1.4 is based on a design from 2006, and has not been improved upon by any manufacturer since. Over the past 30 years there have been huge improvements in computer design tools for optics, and in materials and manufacturing techniques, but it’s unlikely we will see further leaps in lens design and quality at a given price point.
I would go so far as to say that you can’t go wrong buying a high end used digital body, rather than springing for a new one, unless you have specialized needs that can only be met by the new model. I wouldn’t have said that five years ago. For example, it’s likely that the Fuji X-Pro 3 will have the same 26MP sensor that’s in the X-T3. The current but aging Fuji X-Pro 2 has a 24MP sensor. The performance of the two sensors is extremely close. There will probably be some amazing deals on the X-Pro 2 once the next model is announced. Is the X-Pro 2 “outdated” now, in light of what we know about the X-T3? I don’t think so, for most applications.