There is no doubt that there are many professional gamblers that would not have been able to keep themselves afloat without that additional income, and even some of the best players are not immune to this, so this should tell us how difficult it is to achieve that goal, however it is not surprising, as in the case of poker we know it is a game of skill, and with the increase on its popularity the level of skill of the payers has also increased, making more difficult for professional gamblers to profit from it as now everyone is playing at a higher level and they cannot take advantage of weak players anymore.
Professional players probably don't have greed, but they need to think about the risk of completely losing their bankroll because there are fewer and fewer amateurs. This is a risk that needs to be kept in mind. Because of this, they are forced to fight with each other and earn money through rakeback. Eventually some of them will hit a downswing. To be honest, I haven’t played poker for a long time and I don’t understand why it exists, even though I used to like this game.
Probably one of the worst thing that could have happened to professional poker players was the popularization of poker that took place at the beginning of this century, as back then they had a lot of amateur poker players they could take advantage of, but now almost anyone that plays poker today has read a few books about it and it is way more difficult to beat them, and even the small stakes tables are filled with professional gamblers from all over the world that are more than happy to play some poker and make some money each day.