as a grown man you should know better than to take a statement from an ad or from a politician as a promise (remember Bushes "Read my lips" bit?).
In truth, the X1 (or any other keyboard) has a similar obsolescense curve as your PC (or Mac). Sure it has some expansion capabilities, as does your PC. But you can upgrade it only to a point, and then it starts costing you less to buy a new one (even assuming that you can get the new capabilities by upgrading). The difference between the PC and a keyboard (other than the QWERTY vs black and white keys) is that while the PC user will go through the software upgrades which will necessitate the upgrade of the hardware, the X1 (or another keyboard) will play as well as it did when you bought it.
Sure there will be newer instruments, which may have a greater polyphony, or be more user-friendly. But if you liked yours before, it will still do the job in several years.
I stated several times before that I am not a betting man. However, if I were, I'd say that the X100 is likely to be the same as X1, but without speakers, and with a 76 keys, i.e. similar to MS100. It may have a bit more memory capacity, but otherwise, it will be identical to the X1.