The rate of change is at least equal to that of the heyday of personal computers, when faster processors and better displays were released every year. Today, it's likely that a camera will become technically obsolete well before it's no longer repairable. Here's a few reasons why:
- The sensitivity and dynamic range of imagers continues to improve, and rolling shutters are being replaced with global shutters.
- Codecs are also improving, with support of higher bit-depths and bigger color spaces.
- Storage speeds and capacities are increasing, while the cost of flash-based storage is falling.
With things changing so fast, you don't want to get locked into a capital investment in a camera that you can't pay back before it's obsolete. My recommendation is to plan on a three-year usable life for most of today's cameras. That doesn't mean that they'll break in three years, but rather, the state of the art will progress so much that you'll want a new camera in three years, especially if your competitors already have one. So, you need to know how often you're likely to use the camera over those three years.
Let's say that the camera you've decided on costs $20,000, including some accessories that you won't be able to use on future cameras. If you'll use the camera ten times a year over the three years, that means that you'll be spreading the $20,000 cost (plus routine maintenance) over 30 shoots, and the camera will cost you $667 per shoot. (Lenses are extra.) If you're only going to use the camera once a year over three years, it will cost $6,667 per production. A cheaper camera doesn't have to be used as much to justify its purchase, so long as it does everything you expect to need over those three years.
One other important consideration is lens mounts. Even if you're planning to rent most of your lenses, you'll probably want to own some lenses that you use often. You don't want to have to sell your lenses on eBay when you buy a new camera, so you should get a camera with a lens mount that's likely to satisfy your needs in the future. EF and PL mounts are the most widely used today, and are likely to be the most widely used down the road. There are fewer MFT- and E-mount lenses available, but there are adapters and Metabones Speed Boosters for both EF- and PL-mount lenses to fit MFT and E mounts.
If you buy (or rent) cameras with a three-year useful life in mind, don't overbuy based on the number of shoots you expect to do over those three years, and choose a lens mount based on your long-term needs, you're far more likely to be happy with your purchase across its entire usable life and beyond.