Remember also that the company must continue to post Captain slots as available from anybody that got a Captain assignment as a new hire that doesn't want it. So, not only would they have to run out of Captain slots they want to fill for growth/retirements (unlikely in the near term) but they would then continue to have to advertise the previously awarded New Hire Cap slots that were "forced" on pilots as their original assignment.

Since the last vacancy bid had 187 unfilled Captains, I don't think there is any risk of running out of Captain slots in New Hire classes anytime soon. If you are the youngest in the class I would speculate that it is a higher risk that you get a Captain slot "forced" on you than it is that you don't have any Cap slots available. If you are the oldest then you will likely have a choice of 756FO in EWR, 737/A320FO in random bases including IAH or 737 CA in ORD,DEN,SFOor EWR.

I am sure every class will have variations and since 50 different pilots will have 100 different opinions on what is best, I could definitely see that even with the same exact choices available in succesive weeks, one week could run out of Captain slots quickly while the next would have the bottom 10 (eligible) get Captain slots forced on them