Absolutely. But losing an FO with 1000 hours of SIC time means you just lost 1000 hours of SIC. Losing a CA WITH 1200 hours -1000 of FO and 200 of CA time means you lost 800 hours of SIC time. For the system to maintain steady state the average CA before he/she leaves/quits/retires needs to fly enough to generate the 1000 hours SIC to cover their own upgrade and any additional required to offset FOs leaving with their own SIC time. So yeah, CAs are the most limiting factor, but every FO that leaves with OO derived SIC hours is a cost too.