I had assumed that the ADIRU’s inertial reference data from its gyroscopes and accelerometers would feed into the FMS to provide INS capabilities in case GNSS was unavailable.
ADIRU in its minimal common form (some vendors might make a beefier one, who knows) provides the equivalent of old pneumatic "air data central" + artificial horizon and turn indicator gyros.
GNSS becoming "reasonably easy" to add meant that there was way less push to integrate INS after early use in transoceanic flights
GNSS becoming "reasonably easy" to add meant that there was way less push to integrate INS after early use in transoceanic flights