Which generation of Moto X do you reference? The 1st and 2nd generation Moto X supported on Republic’s network are CDMA not GSM devices. The 3rd generation Moto X (a/k/a Moto X Pure) and the 4th generation Moto X may be provisioned for cellular coverage by Republic for either GSM (T-Mobile) or CDMA (Sprint). Whether you’re referring to the Moto X1, X2, X3 or X4, I don’t expect any to go the way of the dodo just yet.
As technology (which is separate from coverage), it’s CDMA rather than GSM that will be going away in the not too distant future (still a few years on that). 4G LTE and forthcoming 5G cellular networks are generally considered to be evolutions of GSM not CDMA technology. Increasingly as 1G, 2G and not terribly far down the road (again still a few years) are turned off, U.S. cellular networks will standardize on technology that evolved from GSM and CDMA will fade away. Most of the rest of the world standardized on GSM and evolutions of GSM technology long ago. The only other large countries where CDMA technology was widely used were Canada (since turned off and converted to GSM and evolutions of GSM) and Japan (whose flavor of CDMA is mutually incompatible with the U.S. flavor of CDMA).
As 5G begins its slow rollout in the next few years, 4G LTE (like 3G today) will become fallback technology. At that point, all U.S. cellular network operators will at long last have standardized on the same network technology and this will be a good thing.
Earlier this year Sprint (which uses CDMA technology) started to turn off service to very old phones. Ultimately, that happens to all phones as they age and the cellular networks advance. The newest of Republic’s legacy phones (Moto E1, E2, G1, G3, X1 and X2) is four years old now (an eternity in technology terms). If one can keep them running, they still have some years of useful life left in them but they won’t last forever.