Perhaps the headline of "300 pilots have left Ryanair this year" should be more realistically be "3,000 pilots did not leave Ryanair this year".
Being realistic, I very much doubt that Ryanair will have any trouble filling the gaps.
My last company lost getting on for 10% of their pilots on some years during the 19 years that I was there despite pretty good conditions (which I am pleased to report that I am enjoying in retirement).
We never had any trouble filling the spaces.