My first impulse was to answer the subject-line question by saying, "No, it's not feasible."
As applied to DSLR-chassis cameras, I think it's pretty clear that this would not be practical. With any small-format camera, lens-flange-to-imager spacing is very critical, and designing a system that would maintain that integrity adds considerably to the size and complexity of the camera. That's one reason that medium-format SLRs with interchangeable digital backs are somewhat pricey, and why the backs themselves are fairly large.
(At my office we have a MegaVision digital back with a 24x36mm imager... exactly what you "full-frame" loyalists would want on the back of your 35mm-format camera, right? Yeah, except that the attachment and support hardware needed to position this sensor take up so much room that the back is the size of a Hasselblad magazine, and that's without a battery or LCD!)
So, I think a user-changeable imager is out, if you want to retain a similar-to-35mm body size.
But then I got to thinking: what about a factory-upgradable sensor?
At first I thought that would be impractical, too -- after all, it's a feature almost every photographer would want, so if it were reasonable, wouldn't Canon and Nikon offer it already?
But then I thought further. A big reason why it wouldn't make much sense to make a DSLR with an upgradable sensor is that usually, by the time a new sensor is available, the camera chassis itself also is ready for replacement. Photographers want new features, faster burst rates, different memory card formats, larger LCDs... it really makes more sense to issue an entire new model with the latest physical upgrades whenever you also upgrade the imager.
A Leica RF, though, isn't in the same category. A lot of people are still quite happy with the physical properties of the M3 just as it came out in 1954, and since then not really a lot has changed: crank rewind, more finder frames, etc. Even the built-in meter and TTL autoflash of the current models arguably are simply add-ons that didn't change the basic characteristics of the package.
In other words, I'd say most DSLR users wouldn't see much value in an upgradable imager because they want and expect the rest of the camera to evolve continuously to keep up to date.
Leica users, on the other hand, like the camera package the way it is, and don't really want it to change other than updating the capabilities inside the package.
So if Canon, for example, were to announce two years from now that they've engineered a 20-megapixel sensor that's a drop-in replacement for the one in the EOS 5D and that any photographer willing to send in his camera and $2,000 can have the new sensor installed, most Canon users wouldn't get too excited. By then they'd want all the other features of the new EOS 55DEFG Xi Turbo, or whatever Canon was calling its current model by then.
On the other hand, if Leica were able to make a similar offer to M8 users, I'll bet most of them would lunge for the checkbook.
Whether it's possible to design a camera this way, or whether Leica would have made any effort to design the M8 this way, I have no idea. I doubt it, since it would drive up the cost of an already expensive product.
But it does occur to me that Leica is about the only 35mm-sized digital camera for which there might even be consumer interest in this kind of thing. Again, it's all because Leica users are basically a conservative lot, who would appreciate expanding the camera's capabilities but not at the expense of changing the packaging.
