This seems to be you asking yourself a question and answering it. What response are you looking for?
I expect you are looking for other social idealists to gather round. I'm not one anymore, but I don't think you should discount those with mixed motives if you will.
Peronally, I don't think that you can divide law students into two strict categories, the social movers and shakers, and the moeny grubbers. Those seeking lucrative career will probably do pro bono and donate to charity. At least I hope so. And as far as changing the world, well some lawyers are senators and some are just on the board of a local interest group. All of us will have political sway as extremely educated people in our society. I hope we all take that very seriously. I was in law school before I realized how much power lawyers have in our society as writers and interpreters of the law.
I can only think of one person at my lawschool who is dead set on effecting social change. She made law review but didn't do the big firm interviews because she wants to spend her summer doing volunteer legal work for the poor. I certainly commend that, but plan on making a contribution less directly.
So for me personally,
I wanted to go to school for three more years (kind of a dork, think learning is fun) but actually be able to make money when I'm done.
I happen to think that achievement and personal success on its own merits is a pretty good gift to society. I'll pay my taxes, have well educated children, vote, maybe write a book, cause some people to think, and genrally try and do excellent work at my job.