I think that it is important to differentiate between greed and ambition. If someone can offer a better word than "ambition", please do. Greed implies that a person's desire for material gain is excessive. How do we measure excess? It is excessive when that desire outweighs other virtues such as fairness, justice or social responsibilities such as spending time with family and friends. Greed doesn't happen at some fixed dollar amount like $10 million or 100 properties. It happens when a person's greed drives them to cheat another person no matter what the amount. Or, when they completely neglect their family in order to earn well beyond any reasonable need. Suppose two people are to split a bill at a restaurant. One person says "You owe $X and I owe $Y" basing the calculation on a dishonest accounting of the bill. Even if the amount that they are cheating the other person is $1, this is greed. It is an example of putting the desire for material gain ahead of justice, honesty and fairness. Suppose a professional asks for a 10% raise on top of their current $150k per year salary. Is this greed if the professional knows that they could get the higher salary by quitting and taking a job down the road? These examples are intended to illustrate that greed is not a function of scale, it is a result of perverse or unbalanced values.
The antithesis of greed is generosity or fairness. The opposite of ambition is laziness or apathy. They are quite different things. Indeed, a person can be both lazy and greedy. However, one isn't lazy and ambitious at the same time.
Remember, "greed" is one of the seven deadly sins of Christian tradition. Whether one is religious or not, the word refers to a specific, negative, human behavior that in our culture, is antithetical to virtue.
@David S. I think that the problem that I'm having with your arguments is that you keep coming back to that word "greed" with a working assumption that beyond X amount, the only possible motivation for continuing is greed. I don't accept that assumption.