For a lot of people, they would upgrade to the latest camera bodies as soon as they are available. I am guilty of this in the past to some degree, but I have now slowed down the upgrade intervals. There are a couple of reasons for this.
In just a bit longer than a decade, the sensor in the consumer digital camera has matured with frightening speed. It's unimaginable just a decade ago that today's sensor is capable of such high resolution and low noise. But, the last couple of years, advances in image quality has been incremental. The resolution of around 16MP is more than high enough for most uses, and ISO 6400 is surprisingly usable. Cameras now can last two to three years and are still very usable compared to the new cameras. So, where do you spend the money allocated for photography? I would suggest lenses.
In film days, we would say that cameras are just a light tight box. It's the lenses that make the images. It's still true today. Camera bodies retire at a much faster rate than lenses. How many 10-year-old digital cameras do you see people use? Hardly any. Lenses are entirely different. I see lenses as an investment, though not entirely in financial sense. I have gone through many Canon bodies, but my Canon lens collection has hardly changed; some of the lenses are more than 20 years old.
True, many people buy cameras not based on image quality, but other factors, such as size, weight, and other features. Manufacturers know this, and will never make a "perfect" camera. there are always features that you want, but lacking in the latest camera, and one of these features will be in the next iteration.