There's an old saying that "A tool is only as useful as the next job it gets used on".
People are generally not tools. But in the perception of an employer, each worker is as much an asset as is a wrench or a printer. The difference lies solely in your upper managements outlook of what needs to get done. The to-do list of every company, large or small, is made up of projects which feed into a bottom line, and those projects in turn are fed by tasks. An employee completes a task the same way a printer generates copies of reports or an assembler builds products in manufacturing.
When we look at what our inherent value to our employer really is, we find that ultimately it is all business. The company's needs define who you are to them, and why they pay you to do what you do. By this definition, there is no real commitment, by either you or your employer, to "promise" to stick around. You can walk out the front door as easily as your employer can let you go.
This is essentially what's known as "at will employment", although I like to look at it as more of two gunslingers in a saloon. As long as each gunslinger doesn't pull a gun on the other, the two can coexist and have their whiskeys in peace. But when it comes time for one or the other to mosey on out, they will tend to watch each other carefully as one of them leaves, to ensure that there's no funny business during the exit.
What this means for today's employee, is that time is no longer a functional measurement of a good job or a good employee. An incredibly great employee may only need to be around for 6 months because he is in high demand. Conversely, a crappy employee may stick around forever because he can't find work elsewhere. The paradigm of the 20-year employee has all but gone away, along with bell-bottom jeans and land line phones, with the new standard being 5-7 years at the most.
The message that comes with this new paradigm is this; if you're still sticking around the same job for more than 5-7 years, you are degrading you employ-ability for future jobs, and potentially you are slowly limiting your knowledge base since you are not doing anything new. Today's work environment moves at the speed of technology, and you need to move with it, or risk becoming a dinosaur.
Unlike tools, humans have the ability to update and redesign their usefulness. Be willing to evolve in your industry, or even into another one.
Because ultimately, your value as an employee relies on what you continue to bring to the table...