I just encountered this again. Sadly, there are people, even professional people, who think this. They get to a place they like and they stay there, sometimes forever.
What kind of artist thinks, “I’m good enough already?” What kind of singer has the idea that there is no further place to go? A bad one, I would say.
Being an artist comes with the responsibility to always be looking for the next discovery and if you sing, that discovery should be at least partly in the voice itself. As you grow, and later as you age, the body is constantly changing. Working to be the best vocalist you can should be an on-going journey for all the time that you sing. It should continue if you become a teacher after no longer performing, as not to keep digging will make you a less dynamic, interesting guide for others.
There are a lot of reasons why people do not continue to work on their singing, most of them not good. Unless there is some kind of debilitating illness or a change in life circumstances that warrants a period of withdrawal from vocal study, a singer who is lazy, complacent, disinterested, lacks confidence, afraid, bored, or simply an egotist, should be addressing the voice as a life task. I have been asked by students, “Why should I work on my voice any more? I like it the way it is.” The answer to that is always the same. “How do you know that you have found all of what your voice can do? How do you know the voice that you like so much couldn’t be even more wonderful? How do you know if you are doing something that isn’t good which could be improved by working with a true expert?” Really, who are you, who is anyone, to think, “I’m already good enough.”
I have been blessed in my life to work with some of the top vocal artists in the world — people making their livelihood by performing as singers — and none of those people (zero) was “content” to rest upon his or her laurels. The more professional they were, they more willing they were to work on being not just good but excellent.
If you have run out of ways to work on your voice and on your singing, you need to acknowledge that it is completely unnecessary to be stuck there. Skilled teachers will find ways to take you to the next place, to challenge you to grow and to find new ways to be artistic when you sing. You might have to look to find the right instructor, but if you seek, you will find.
Never rest on your past or current accomplishments. Doesn’t work.