Cadet schemes are fantastic for boosting numbers and solving short term problems but a previous employer I worked at got caught out, they had a continued stream of Captains leaving very quickly and they actually ran out of internally suitably experience FOs to upgrade. They had to cancel flights. Great news if your a regional FO, upgrade could be just around the corner.
Pilot Shortages could be company induced also. I was part of a mob (Airbus) once who cut pilot pay by 10% to fund a decline in profits. Half the pilots left cost the company ten fold it was going to save in labour and cancelled half the flights. They paid foreign pilots some incredible figures to assist getting flights operational again. That pilot shortage took about 2-3 years to level out.