You can't. It is not a quantifiable attribute.
Aren't you trying to quantify it when you set up a hypothetical where two people from different backgrounds have the same willpower?
Why should it be easy? Life is complicated. The obstacles people face are many and unforeseen.
It's as complicated as you make it.
For any individual person, I generally wouldn't say that anything is not achievable. However, in the aggregate, it is quite easy to say that things will not be achievable by the majority. If someone were 5'3" tall, I wouldn't say that it is impossible for them to start on a professional basketball team. However, if you were to ask me if all 5'3" people were capable (assuming they all put forth all of their effort) of playing professional basketball, I would be a fool to say yes. Even if there were many more professional basketball teams, the fact is that if you are 5'3" tall, you are suffering from a major handicap when it comes to professional basketball. Not everyone, despite their respective "best efforts" will succeed. That is also true for people 7'2", although I think we can all agree that someone 7'2" has an easier shot (no pun intended).
I think the best you can say is you don't know, where the aggregate is concerned. You can break things down into groups like we're doing. Take all the people who want to be pro ball players. What do we know? We know height is valuable. We know as the height of our pool decreases the harder it will be for those individuals to make it pro. You can see the parralel to success in attaining wealth. The less conducive your background is to your goal the harder you will have to work to attain it. I simply believe the group of people that attainment is impossible for is very small.
Try not to stereotype. We are idealists because we believe it is possible to create a world where people have greater opportunities to achieve their goals. However, we recognize life's limitations, even if we want to reduce those limitations. We are not blind and stupid.
Then you will be sorely dissappointed. You can't create opportunity. It's always there. They will be harder to find for some then others. What you want to do is create ease of opportunity. The strong members of society are those that overcome adversity through their willpower and hard work. We've been throwing around names like the children of the Hiltons and Trumps. They are certainly wealthy, but are they valuable, strong members of society? Not really. Strength comes through difficulty. Strong societies are those that experience adversity and perservere. So what are you really doing for society by eliminating adversity?