Yes, we've heard all that before. Many choose to update their OS primarily for security and software compatibility (as our macs do other duties too at home).
Looking at Avid's
End of Life Dates, the FastTrack C400 supposedly receives support until 2/11/2018. It is not unreasonable for C400 owners to expect compatible drivers until then.
If this is not the case, you can expect some of us to find another audio solution.