Or, imagine you are on the recruitment team for narrow body f/o's and you have two 25 yr old candidates;
Candidate A did five years as a Rex f/o then upgraded and has two years command multi crew and a checking history that means something to you when you look at it ( because you are familiar with the airline and what it produces)and see that he or she has consistently scored very well in all checks and in addition has participated in annual HF/EP classes etc for seven years. There are no red flags and the motivation and attitude is appropriate.
Candidate B has more command time, much less multi crew time, has worked for seven different outfits on all the usual suspects (206, 210' 310, Chieftan) has produced check forms from Jo Bloggs that simply say " good standard" and has not been a part of any training matrix at all but has motivation and attitude that seems appropriate.
Who is the greatest training risk to your narrow body airline?
I know who I would choose. That is the reality.