Working at a single place for 10 years and having 0 connections to other companies is not going to put you in a good place to find a job. I imagine they would struggle to find a job regardless of whether or not they quit.
I ask in part because I've been at my current employer for 9 years -- should someone in my spot be looking for a new job?
Try interviewing and seeing what you get offered.
Unless you manages to get on the gravy train, in which case, congrats!
I think that what hiring managers want to see is that you fully exploit the opportunities available to you. That means that if you're in a good employer and given a lot of responsibility, you stay. If you're in a dead-end job where you're not learning anything, you leave. If it's a good company going through a rough patch, you stay. If it's just a bad company, you leave.
Oftentimes that means you'll have stints of varying lengths on your resume: you may have a few of 1-2 years when starting out to find your footing, then a longer stint of 4-5 years at an industry-leader, then a shorter stint at a smaller company where you're given more responsibility, then a startup, and so on.