This is another popular cliché, and I admit, I'm not fond of this one either, but what if we thought of it as lifetime learning? Does that make it any better?
The hard part of this, in my opinion, is two-fold. One more insidious than the other.
The first is humility. No matter how far along the journey we are, we still have more to learn. That is the beauty, and frustration of music. Perhaps the most interesting part of it all, is that we do it to ourselves.
I read somewhere that mastery of a subject is the ability to envision just beyond your ability to execute it. The more we learn and hone our craft as musicians, the more our imagination can take it just a little bit further. That phrase, just a bit smoother, those vowels, just a tad more connected and balanced.
And so the cycle goes around again...learning more, practicing the specific skills needed for those phrases, and so on. This will make a music practice either very exciting, as it never ends, or somewhat frustrating, because no matter what heights you lift your performance to, there will still be further to go.
I mentioned a second point, and I said it would be more insidious. So it is, if you ask me. What's more, it goes against our nature, if one subscribes to the concept of such things as human nature.
Simply put, we like to be comfortable. We limit ourselves by choosing repertoire that remains in our comfort zones, be that of range, genre, or style.
We needn't necessarily work repertoire up to performance standards if it's not in our wheelhouse, but practicing those pieces can inform us on how our voices operate; give us a different view to our singing practice, and ultimately level up our chosen musical art form.