chris lz,
1) AINS is inherently less trustworthy than ground aids
I would say so. It was developed for long range navigation, and was certainly trustworthy enough for that task. But I have never heard of an AINS approach to land procedure.
2) The primary reason is because ground aids are calibrated regularly.
That would be fair comment Normally if there is any problem with the aid it will stop transmitting its identity code and cannot then be used.
3) Therefore, AINS should not be used a substitute for such ground aids
Not for a let down below route MSA, especially not after a long time without any ground based aids to confirm its reading, and any doubt as to a visual positive fix.