I would have thought that mentored schemes are aligned with the strategic direction of an organisation and that probably means being in alignment with forecast retirements and planned fleet expansion.
They probably are not a very good way of dealing with general capacity shortfall which seems to be what has been experienced for the last couple of years. However, when an over capacity is realised so the projections are changed and so the aircraft order options are reduced and 'in-flight' mentored students chopped.
I don't see mentored schemes as a replacement for more 'ad-hoc' recruitment. Probably the case that both are essential for smoothing resource requirements.