Congrats on graduating. You make a good point, however, I'd like to make some counterpoints:
1. With the current financial position of the federal government, and the projected situtation 10 years from now, it may be a gamble to assume that they will keep this loan forgiveness program in place. Some things are going to need to be cut, and something like this might not survive. I say the same thing to people who think the Roth IRA is such a no brainer. It is, assuming they don't change the rules, and that is a big assumption.
2. Also given the future financial condition of the federal govenment, and the looming devaluation of the dollar to deal with huge deficits, accumulating money now has to be a priority, even if you are not materialistic as the OP is. With a public interest law job I doubt you'd be able to save a dime.
Now if it's something the OP truly, truly loves, I'd still say go for it. She won't strarve. But there are tons of people who go to law school thinking they will do public interest law only to find out that 1) it pays poorly; 2) there are few jobs; 3) it's not what they thought it was going to be. The OP doesn't want to get into that financial hole (and it will be hole, especially vs. the opportunity cost of giving up her current job) without thinking it through.