There are parachutist managers. Just because someone is parachuted in, does not mean that they indent on leaving. Whether they need to leave later or not, depends strictly on the way they execute the transition.

Also, badly executed transitions can hurt the bottom line a lot, and for a long time, so they are kind of frown upon, regardless of how much the transitions themselves may be necessary. Just like you have "figured it out", most of the employees know that, too. People are not dumb, especially when it comes to their workplace.

Finally, the bag of cash is necessary, because often after a badly executed transition that person won't find any further high-level management jobs anymore.