If you know there is a resource shortfall , you need to remove a sufficient number of flights to protect the rest of the flying programme.
You decide which flights to cancel and create a pool of pilots who can be assigned as and when required.
It would be nonsense to wait until there is a problem with a specific pilot
and then cancel the flight they happened to be rostered on, otherwise you could end up cancelling a service 100% full and operating one only 50% full.Or operating a flight to somewhere easily accessible by other means like Beauvais and cancelling a flight to Tenerife.
You manage the resource you have to operate the flights you want to operate.
There is a big incentive for Ryanair to get the cancelled flights published asap,it will save on compensation and in addition people will be very reluctant to make a new booking on Ryanair up to end of Oct while there is a possibility the flight
may be cancelled.