What might be more of concern is if the vast majority of losses fall into the 'movers-on' category. That identifies that they don't view their company as a medium/longterm career provider. That's disappointing. It reflects on motivation, loyalty and personal investment. It also causes the in-house training dept. a lot of work and dilutes the company culture in higher ranks, or leaves a short-fall of suitable candidates for promotion.
I understand you're not saying that with your Ryanair hat on, but it's not really strange if people don't see it as a long term career? Unable to get a mortgage, unable to settle down at any location, tax/social insurance issues, not knowing how many standbys the winter will offer, years ticking by without any accumulated pension (unless saving privately), not knowing when the next home base opportunity will open (or stay open, for that matter), painful commuting, threats of 5/3 roster. It works for some, the prospect of PIC time is nice. But there shouldn't be any surprise why experienced FOs in late 20/early 30 that are ready to settle down with their partners, perhaps have kids, look for something better?