What leads to professional success as a musician?
Talent matters. The ability to hear and feel and think. Digital dexterity. Work. Perhaps luck, or chance, or random events play their parts in many careers. And there are other elements of “talent.” Some artists interest us. The sounds they make compel. We want them around, we want them to be part of our life.
There are so many young musicians of real ability and achievement who do not make commercially successful musical careers. What happens? Are there just too many of them? Do they lack perseverance? Do they lack what Marian Seldes described as the performer’s necessary capacity to absorb disappointment?
Certainly I recollect many pianists who impressed me tremendously in their teens and twenties who have now disappeared from view. 
I’m thinking of musicians of very high order. It can seem sad, arbitrary, or wrong.
A manager or teacher may have prescience about who can make it. Often, I believe we are too careful. We may just be encouraging those who are most like us. Can it be we are trying to perpetuate our musical world, with all its limitations?
A real success would be the youngster who can blow the lid right off.
Persistence must be taught simultaneously with any curriculum.
Vicci Johnson/Retired St. Paul Public School Music Ed
Maybe there’s just not enough room for everyone. Look at the field we’re in. Is their enough demand for piano soloists to support every talented 20 year old in a conservatory having a major career? Of course not.
This is a natural thing, like every talented basketball player not making it to the NBA, or every talented writer not producing a best seller. Realizing it hurts, to be honest, but if people really love what they are doing, and can sacrifice financial stability for awhile (which is extremely hard, especially with student loans from music schools that are so much more than aspiring musicians can count on making,) then a life in music is possible.
Who knows how many people you are thinking of who have established smaller careers in music that are necessary and make them happy? Or how many people went out and found something else they loved?
To add onto what William said, how many players still play for the NBA but aren’t household names, some even in their hometowns if they play enough of a supporting role. There’s still a difference between them and those that never make it to the ‘professional’ level. Whether anyone outside the basketball world will necessarily notice.
I agree with William. I don’t think every extremely talented pianist is meant to work as a concert pianist… probably those people went on to become doctors and lawyers and engineers and teachers… and there is nothing wrong with that.
I would have a problem with someone who says they really really want to do it, so far as declaring the major, but refuse to put in the practice hours in and never come to lessons.
Sadly, I do know a few like these.
The stage of life is also a factor that leads to success. You may encounter many talents even in old age.