In an at-will state, employment is never truly secure. However, in an at-will state, it is not just the new employees who are at-will, it can be most or all of the employees too.
If the company is going through hard times, they may need to lay off very important people too. If there is politics going on, it can result in collateral damage of people at all levels. There really isn't any such thing as a completely secure job, and many good, competent, and advanced people end up laid off.
On the other hand, while most people don't work 30-40 years at the same job and then retire, there are still many companies and jobs where people do work for many years, even in at-will states.
When you should (generally) not be looking for a new job in an at-will state:
- You've just started at that job, and it seems like a decent one.
- You're happy with your work, your co-workers, your manager, and the company seems stable.
- You haven't worked at the job for very long yet.
When you should be looking for a new job in an at-will state:
- When the company appears shaky, is talking about layoffs, or you're having difficulty with the work or people.
- When you've been at the job at least a couple of years and you want to see what else is out there.