So if I understand you correctly, the laser gyros are practically error free and the modern accelerometers, while being very accurate, aren't 100% accurate (causing the eventual map shift if not updated by an external source via VOR/LOC/DME or GPS through the FMC).
So what about the mechanical counterparts found in GA aircraft, the ones spinning at 30k RPMs? Can the attitude drift there?
@OBN:
No, the external sources will never update the actual internal position provided by the ADIRS. That's done in the FMGC in the Airbus. And definitely no GPS update will be used for attitude data (and that's what this post is mainly about).