Of course existing crew need to be utilised to 100% before more crew are added to the roster but consider this:
The crew at the top of the list, first cabs off the rank will need several months to requalify to 737. Most haven''t flown it for 7-10 years so re-recruiting will need to run out months in front of utilisation of existing crew. Once existing crew get to 100% it's too late to start retraining. And there is finite sim availability.