I don't know where the "rapid expansion" idea comes from. Jet2 have done anything but that over the past few years - adding one or two aircraft (maybe 4 next year) at a time.
There is a great deal of guff being talked here at the moment. Certainly all of us at jet2 know people who are leaving, and that contrasts with everyone staying put over the past couple of years - but it certainly isn't a rush for the exits, and is well within the normal turnover of good people you would expect in any workplace. The overall experience levels are much higher than when I joined 5 years ago, and loose talk about safety being affected is, to say the least, pretty irresponsible.