You ignored the first part of my post; that was where my most important point was.
Because I don't understand how that analogy is applicable.
All right, so we know that we all have the capacity. No matter what I do, that does not change my capacity, and it will not change the capacity of anyone else. How does changing my behaviour in secret compell other people to behave in a similar fashion?
What you do doesn't change your capacity, but it is a measure. If your thought process is literally "I know I could behave better, but I don't care because it doesn't change anything" then you have in fact proven that your capacity for good behavior is proportional to your immediate perception of return on that investment. Apparently, if it doesn't directly make the world a better place, then personal gain is favorable.
Unless you believe that somehow you're incredibly special and very few people have a similar thought process to yours, then it is safe to assume that a considerable portion of the population will operate in the same fashion as you. If those people behave selfishly any time they're not convinced that their decisions will have broad implications for society, then that's a lot of selfish behavior, the sum of which DOES have broad implications for society.
Or if you don't believe in the above, then I see two directions you can go. Perhaps you believe that if you hold yourself to a higher standard, that you'll just be better than everyone else, giving up on personal gain while everyone else continues to behave selfishly and it will all be pointless in the end. What a pretentious view. Or you could go the other way and assume that enough people hold better standards of behavior than you that your isolated acts of selfishness won't matter. A world made of good people can support a few bad ones, right? This one doesn't look so good, either.
The following is my personal perspective, and how I try to live my life.
Honestly, I am abhorred and uplifted by various instances of human behavior every single day. I don't like the current state of the world, as I think those who behave selfishly end up in positions of greater influence. However, I desperately cling to a belief that society can reach a higher standard. I could not go on living in this world unless I did. I see society as the sum of all human behavior. I cannot maintain a belief that that sum can reach a positive unless I can succeed in maintaining a personal standard at the level I wish to see. Even more importantly, that state of society defined by a standard of responsible, mature, and compassionate behavior X literally cannot exist until a large enough portion of the population actually maintains X as a personal standard. The first step to achieving this is to achieve it myself, and the more people realize this, the more capable we are of moving forward.
The best example I know is the earlier one I mentioned regarding pacifism. It is literally impossible for a world where people do not use violence to exist if I make use of violence. People say that such idealism is impractical, because it relies on an ideal situation to operate. They're completely missing the point. The point is to align oneself with that ideal situation to allow it the potential to exist.