Most of those leaving Monarch are close to the bottom of the seniority list. Monarch would be their first airline, and so are jumping to their career airline aka BA, EK and VS.
I'm pretty sure they are not leaving Monarch because they hate it, they just can't see a career at MON. Majority of people from other airlines join and tend to stay as they see an improvement in T&Cs.
However expect to work hard. With an increase in winter flying, seasonal variations of the past are reducing.