I think if just look at it in terms of value, I think digital cameras dont hold their value well. An example is the Nikon D1x which is worth a fraction of its original price 2nd hand. The M8's 10 mpixels is going to be similar to the latest mid-range to low end DSLRs soon (e.g. Nikon D80, Olympus E-400), if you don't consider other IQ factors(no AA filter, Leica lenses etc).
If you look at it in terms of the professional price bracket, there are rumors that the new Canon 1-series will appear soon, rumored to include features like improved colour, dust remover on the sensor, smaller body, improved autofocus, improved resolution, together with new lenses).
I think what the M8 has going for it is that its the best RF digital on the market today, its fun using RFs (simple manual control), the resolution is already adequate for the kind of print sizes most of us will be printing at and the M8 re-produces the clarity and unique lens characteristics of leica lenses.
Hopefully Leica will make the sensors upgradeble, because its hardly likely that other portions of the M8 can be improved i.e. meter, autofocus like canon/nikon, so the only changes would be the sensor; and its unlikely that Leica will change the design of the M8 body.