I don't think there is any flattening of the learning curve.
But there may be a balancing of life's priorities. The person who is really good at something usually puts a lot of effort and thought into that activity. So the thought could be if after 5 years I'm just average, unless guitar moves up the scale in priority, I will probably always just be average. For the person who in their first 5 years really excels, unless guitar moves significantly down in their balance of priorities, they will probably continue to excel.