Kind of sad we turn these great pieces of technology obsolete after just a few years. I don’t remember this upgrade frenzy with film cameras.
I think there's an obvious reason for this difference between film and digital cameras.
Really, the ability to take great pictures gets down to two things: The optics, and the sensor and supporting electronics, for digital cameras, or the film, for film cameras.
To whatever degree the technology of film itself advances, you can always use the most modern, technically-advanced film, even in an old camera. My 1972-vintage F2 can take pictures every bit as good as what you'd get using a brand new F6. Same film, same optics.
Electronic technology has been, for a very long time, advancing very rapidly. This applies, of course, to digital image sensors, and to the electronics that support them. Whatever the very best there is of such on the market at any given time, there will be even better in a year or two.
In the time span between my F2, and the F6, which will take equally-good pictures on film, we've had the Kodak DCS 100 come along in 1991 as the first DSLR, at a price of $20,000, with a resolution of only 1.3 megapixels, the Nikon D1 in 1999 1t 2.7 megapixels, up through the current batch of 24-megapixel and a few 36-megapixel cameras. A year or two from now, I am sure we'll be seeing fifty megapixels or more, perhaps even a hundred megapixels. Of course, there are other factors beside raw megapixels, that affect overall image quality, and these are improving as well, at least as rapidly.
The only way that a digital camera could not be doomed to obsolescence within a few years would be for the sensor and supporting electronics to be interchangeable, so that you can just replace those rather than replacing the whole camera. As far as I know, only Hasselblad does this, on cameras that have prices today in the tens of thousands of dollars.
Unless you're buying a Hasselblad, you're just going to have to accept that any digital camera that you buy today, will not be up to the technological standards in pace a few years from now.