I wonder if part of the problem or challenge for us all is deciding whether or not something is obsolete for us, or not?
BB,
That´s an interesting question you present. I think most of us are replacing cameras, not because they are obsolete, but because we want something new or different. In that sense, an interchangeable sensor camera would do us no good, unless new features could also be implemented as they became available, through firmware updates or otherwise.
As I have said before, I was a late adopter of digital, buying my very first digital camera in late 2004, the original Canon Digital Rebel. I only used the Digital Rebel for about 9 months, as I bought a 20D when it came out. The 20D was a milestone in DSLR development. As compared to the Digital Rebel and 10D, you did not have to wait three seconds (which feels like an eternity) for the camera to wake from sleep. The camera was quicker and more agile in every sense, and the image quality set a new standard for APS-C DSLRs at the time.
That was the only time I have replaced a camera because I thought it was obsolete. Even today, I do not regard the 20D as obsolete. The image quality is still first rate, and even though 8.2mp may seem low, it is plenty for most purposes and very large prints may be made with judicious use of uprezzing.
To get to my point (there is one, I promise) I did replace the 20D four years later with a 50D, but not because I thought the 20D was obsolete. Rather, the 50D had features that I wanted that were missing on the 20D, such as live view, automatic sensor cleaning, and the larger LCD. The extra megapixels are nice to have (mostly for cropping) but in reality I cannot tell the difference in my prints from the 20D and the 50D (up to 12¨x 16¨). There is no significant difference in image quality, as far as I can tell.
I guess I am trying to make two points here: (1) digital technology is mature enough so that obsolescence may come at a slower pace than before; and (2) if we are upgrading based on features, an interchangeable sensor camera body will be of no benefit to us.
P.S. I gave my 20D to my sister, who is an artist and director of an art museum. She has been putting it to very good use.
Regards,
Antonio