How is it obsolete? The x100 is special in that the sensor and lens are both perfectly tuned to one another. The viewfinder is only for the 35mm lens, and therefore is fantastic too look through - big and bright with just the right amount of space around the frame lines . Interchangeable lenses would mean that the viewfinder would be compromised - possibly just no optical finder, only an EVF. On the x100, the EVF is rubbish compared to the OVF. It would mean that a similar 35mm f2 fujinon lens would be much bigger (on the x100 the lens unit goes very far into the body - specially for that particular sensor), and possibly not as optically good.
There are more things too - more compromises that are to be made when you have interchangeable lenses. As I said before, I would still be using my x100 despite the interchangeable system, unless they can avoid all these compromises - which is doubtful.