Yawn.
It is the same story every time an expansion comes along. There are not enough trainers to do the task.
You can't just upgrade another dozen training Captains in a month or two and then solve the problem. That is just like being short of "pilots" and simply hiring S/O's. The Training team need to be expanded in depth to cope with an increased task. This needs to take place before the new basic trainees enter the system, since you need training resources to train the new trainers.
The result is that the available training sectors limit the expansion for the whole airline.
If you need one new pilot....
To upgrade an S/O, you first need to factor in the training requirement of his replacement S/O. That is 10 training sectors, mostly ULH. In addition, about 20% need more training.
Then his/her 60 sector JFO course needs to be accounted for (even though about 30-40% check out at 40 sectors)
To recruit a Direct Entry F/O the programme is 30 sectors of line training. Approximately 30% need some extra sectors. The record is 27 extra. (still held by the 74F!)
The number crunchers calculate that in balance it is best to recruit DE F/O's onto a base during an expansion (when training sectors are at a premium). This is even before the costs of based and expat pilots are considered.