Originally posted by Jerome
This quesiton is quite tricky. When a new DSLR model comes out, the old one almost always become obsolete quickly in the market.
I tend to agree more with Ian's assessment. Chances are that we will eventually reach a certain resolution level (15 megapixel? I'm leaning more towards 22 megapixel, but the point remains the same) that will clearly suffice for all but the most demanding photographic applications, and this, together with the hopeful eventual standardisation of sensor size (whether it be 35mm or the Olydak "four-thirds" or whatever), will eventually greatly diminish the intensity of research into achieving greater resolutions, with far more effort being subsequently devoted to lens and body designs. In such a case, we ought to see massive declines in prices, hopefully eventually to current 35mm SLR levels or even lower.

It's like computer. Once Pentium IV is out, Pentium III will still sell for a short while after which it dies completely. But how many of us need a Pentium 4 at 2 GHz?
Indeed, but the human race's need for computational power is still exceedingly great right now, considering the many amazing applications and innovations that would be possible if we just have enough computing power. The human imagination apparently knows no limits.

On the other hand, as far as photography is concerned, there will come a point when we can say "enough is enough" simply because of limiting factors like the resolving power of our eyes. This is along the same lines as why 24-bit color is already sufficient for human eyes and we don't really bother with trying to produce displays with even greater color resolutions. True, some specialised applications may demand more detail from our photos, but such "higher-resolution" cameras will likely be something like medium format and large format cameras today: they're there if you need them, but the vast majority of everyday photographers are more than happy with the "standard" offerings.

To offer a very imperfect analogy, but one that suffices to illustrate the general principle at work, also from the computing industry, remember how dial-up modem speeds increased very rapidly in the early 1990s? We went from 2,400bps to 14.4kps to 19.2kps to 28.8kbps to 33.6kps in a matter of years, with successive generation coming out at pretty high price points. Eventually. though, we reached a limiting factor (the capacity of telephone copper wire) and became stuck at 56kps, and once this limit was hit, prices dropped so rapidly that the price of a 56kps modem card is almost negligible these days. As I mentioned, this is a very imperfect analogy, because in this case we still needed greater bandwidth and so moved on to other technologies; in the case of photography, however, I believe we will indeed be able to say "enough is enough" one day.

Just my two pennies' (ok, maybe two quarters') worth.