You have to be present in order to improve. That gets harder the longer you study one thing because you must continue to stay interested, finding new ways to improve and develop. Most hobbyists will never do this, so they brag about their 30+ years of training, but really have done less then a serious 3 year student.
It helps to have teachers and peers who will push you. If you stay around long enough, you'll eventually become a senior student. At that point, it's easy to become complacent. Who will call you on your mistakes? If the answer is no one, it's time to move on.
(This is a particular challenge for dojo chos, and why I think running your own dojo before you're pretty senior is often unwise.)