My perspective is that for this type of artist who is operating at the highest level (according to themselves) they are always capable of making an even better masterpiece than the one they made before. So the potential is always there to raise the bar over and over again, but obviously we can only see those results if someone is paying for them. And sometimes these new projects will cost a huge amount of money, and no buyers can be found, and that unrealized potential just dies with the artist.
Discussion
So besides all the great artworks that got destroyed over time, I also think about all the great artworks that could have been made if there had simply been one more patron to fund the next project. In retrospect, we would pay anything to have extra masterpieces from the great artists in history, but at the time that this is possible with art patronage, the cost is relatively tiny.