Mentoring of instructors is just as important as mentoring junior line pilots.
Indeed.
It really comes down to the quality and attitude of CPs/CFIs and internal QA systems. Well do I recall my introduction to airlines at Ansett, many years ago. Like most young, overconfident chaps, I thought I knew far more than I did ... it didn't take very long for me to realise that my confidence exceeded my competence, somewhat, at the time.
The other concerning problem is the lack of general study undertaken by the new chums coming up through the ranks. In the olden days, we pored over local and foreign crash comics with wide-eyed horror .. and learnt a bit with each tale told. These days, I see far too many of the new folk who, quite clearly, have no interest in anything beyond what is put in front of them. For example, in a theory class, a year or two ago, I had one young lady remonstrate with me for daring to introduce some trivial, but quite relevant, mathematics when discussing navigation computers and 1:60 things. She was interested solely in what was in her assigned text book. Go figure ?