Plenty of people tell you that, whether you want to listen or not is a different matter. People constantly advise others to continue to gain skills, get a degree, get certifications, make sure you learn new stuff every year to stay relevant in whatever field etc

A lot of motivated people understand that they are replaceable much more than your middle of the road worker does, usually why they are always making moves and thinking about themselves above the company's interests.

Your value can and will drop if you don't actively stay on top of it.