If your average head is there 5 years, something is wrong. Are the people leaving voluntarily, or being let go. If voluntary, there is a problem with salary, benefits, and general culture of the organization. If being let go, there is probably something the company is doing to control expenses. Like qualifying for some benefit (extra vacation days) or vesting in something like a retirement plan, and the copmany is paying the penalty for the result.
Regarding the 'extra effort' comment. I work for a large company. It used to be a team effort between the company and the employees. The employees would do whatever was necessary to be successful. They liked their jobs and the company. When times slowed down, the company would invest in internal projects to make the product better, the process better, special training opportunities, etc. Found things to keep the employees employed, busy, and ready to do it again when business turned around. Not any more. The company now wants more from less, and when thing slow down, the people are out the door. And the company doesn't want the back when things improve. The company wants new hires at lower salary, lesser benefits such as minimum vacation time. The result is the best people are continually shopping for a better job and better company. They leave. The company is left with the mediocre or less.