Sure, but when we're talking upgrading within the brand and camera type, why make it harder for the customers?
I'll use a crappy analogy of say, wanting to drop a slightly bigger LS into a Vette, but then hearing that in order to enjoy that slightly better engine you MUST get a brand new Vette as it's simply and completely incompatible with yours.
I totally agree with you on that point. Unfortunately, I don't have enough DSLR history to talk without fear of contradiction, but it seems Canon made a break in their lens line ups and compatibility a few years ago, such that Canon fans had to make some hard decisions to stay with the brand or not. Again not having the whole history but I've read a number posts in different places telling of Canon fan defecting to Nikon (and others brands no doubt) on this very point. So basically speaking Canon has priors in this arena of culling the heard for business reasons, forcing the most loyal fans to upgrade.
However, the same thing can be observed with the iPhone. This is by no means a statement about the iPhone or Apple because today Apple makes some fantastic products. However, I have just a tad of marketing background and have to say Apple has no fear stretching the limits of fan loyalty. I defected from Apple years ago - my MacPlus is a novelty item sitting in the back of my closet. Apple Mac's have matured tremendously over the years, but back in the 1980's it was Steve Jobs at his finest with marketing hype and very little to back it up. In those days, a major portion of my income was custom home design/drafting and I needed to step up to CAD. The MacPlus was near impossible to upgrade, and the newest model Apple offered with expansion capability cost a small fortune. I had purchased for my small business Macintosh compatible hardware, accounting software, database software, Excel, Word, etc., to run on my MacPlus, but after I polled the major CAD/CAM software developers, I walked away from Apple and bought a PC. By comparison they were like trying to make fire by rubbing sticks together but true production applications were available, allowing me to upgrade for 1/3 the cost vs Apple's marketing treadmill, hoping to get another ration of technology from Apple.
Seems what you're talking about is analogous to my experience with with Apple. Again, Apple makes some great products and their fan loyalty is justly deserved.