The department cannot get rid of mission critical maintenance. If you do that, your company dies right there and then once you're even minimally unlucky. The newcomers have a delay where they need to be trained, which is typically measured at at least a year.

The new guys get removed first. Ones working on shiny less critical processes. Shiny and critical generally happens in startups.

When legacy is being replaced, old timers generally see the writing on the wall and move on as well as retrain.