BigBus,

Nobody will be stuck as an FO indefinitely, unless a particular FO does not have the experience or performance to upgrade (you need both). We always try to upgrade in seniority order, but some FOs just are not ready yet. Putting them up for upgrade before they are ready knowing they will probably fail is not helpful for them or anyone else. When they are ready, they will upgrade. If there are not enough who are ready to go, they'll put in street captains.