Internally the CC121 is a simple Yamaha MIDI device that uses the same driver as all other Yamaha MIDI devices.It works even out of the box under Linux. So I doubt that it will stop working anytime soon from an operating system perspective.
My biggest concern is that the CC121 will not be supported by future Cubase/Nuendo/Wavelab versions. These applications require dedicated add-ons to support the device and the APIs are not open to the public. This means that only Steinberg can keep the add-ons up to date. If they don’t do it then the device turns into an expensive doorstopper unless you refuse to update Cubase or switch to Reaper.