BCNS,
FR don't have the requirement for DECs that existed 5 years ago: there are enough SFOs ready for CU within the company to fill the majority of available positions in the foreseeable future. That said, guys with good command time on the 737 - 300 - 900 are not necessarily being rejected out of hand.
Nevertheless, the company prefers to take shiny new cadets ( first rungers ) and train them to the RHS because they don't come laden down with other airlines SOPs and cultures.
That is the choice of the management: it has little to do with the cost of the TR or any "profit" generated.