Maybe you should clarify if this discussion is about digital that's available to amateurs to mid-end professionals, or digital available if you have more money than god
The liquid-cooled CCD cameras that astronomers use to make telescope images are great and all, but I don't think anyone on this board can afford one. I know none of the astronomers can; that's why they have to sign up for months or years in advance to use one.
We're starting to see some slowdown as digital matures. We may even be running into some disadvantages to upping sensor size. There are some cases where the new 50D is outperformed by the 40D (not many, but a few). As you keep shrinking pixel size, you lose some quality (it's why full frame cameras out perform APS-C, why medium-format outperforms full frame, etc). It requires more engineering (and much more cost) to try and overcome these limitations.
It's not just cameras; even CPUs are hitting real limits. The move to multi-core processors has partly been necessitated by the problem of running a CPU past 3 ghz. The chips are running too hot. It becomes exponentially more expensive to build each succeeding generation of fabrication facility. And even when you shrink die size, there's still the need to try and lower voltage in order to control heat, which introduces problems of noise.
This is certainly applicable to cameras. It's not just a light-sensitive CMOS sensor. There's a custom processor alongside that has to make an image from it. And digital camera manufacturers can't afford the latest fabs. I believe Intel is still the only company with 13um fabs up and running and they aren't doing work for anyone but Intel. Plus, the aforementioned factors afflict camera manufacturers. If you were willing to lug around a 20 pound dSLR with a nitrogen cooling system and multiple processors, you'd have incredible image quality. You'd also have to spend a small fortune. And your arms would look like Popeye's
You also have to figure in what is asked of the object in question. As for the Checker car, the first thing to note is that only the body was "indestructible". All the other parts wore out, like all mechanical parts. But we started asking for things from our cars that a Checker can't provide, like gas mileage. It doesn't do you any good to own a 6 ton, indestructible car if you can't afford to put gas in it. Plus, a lot of those cars stayed running because they were used for business. They received regular maintenance and were kept in service to pay off the investment. Ten years from now, there will probably be more 1Ds Mk3s around than XSis, even though more of the latter are built. Part of it also has to do with behavior; we became a culture that replaces its cars every three years. It doesn't make any economic sense to build a car that lasts forever, no would enjoy it.
Figure that it's easier to manufacture something new than repair something old, why shouldn't you buy new stuff? It's been wasteful in the past, but we have also gotten better and better at recycling old items and breaking them back down. Fifty years ago no one would have imagined an item that was 90% recyclable. Most people couldn't even define the word recyclable.