The issue is that there are too many of them. Let them retire please!!!
I think the issue is not that they are too many, but that demand for them is too low.
For example, I can use a $300k salary center, but I don't want to buy one because their salaries a too high!
There lies the biggest problem with the economy. Since player salaries increase with attribute improvement exponentially, players with skills above 16/17 become economically inefficient. The difference is quality becomes too small compared to the difference in salary. Therefore, it is way more efficient to buy 2 150k players, than 1 300k monster.