When an employee leaves a company, whatever the circumstance, it creates chaos. Who will cover their duties? Will they be replaced? Will someone be promoted to fill the gap? When the person that goes is a manager, things get even more complicated because the many people under the departed manager are left up in the air.
If the company hires a new manager, will the day-to-day operations be worse than before? Or will their arrival bring better operations? And if the company promotes internally you may find yourself now working for someone whom you are friendly with, but unaccustomed to working for. And what if you are the one promoted? How will you respond to the situation? And how will your colleagues react?
Power vacuums are a messy business and should be dealt with quickly and definitively. Routines should be re-established (even if they are modified) so business can continue. The alternative is a depressed work-place — and who wants that?