PSLF has been written into the MPN for all eligible federal loans since 2007. There is nothing to worry about for people who have already taken out loans. The MPN is a binding document on both sides. They would get sued by tens of thousands of people if they tried to go back and retroactively cap the amount forgiven, as no such cap is mentioned in the MPN. The exact wording has changed slightly over the years, but it's always been something similar to,
"A Public Service Loan Forgiveness program is also available. Under this program, we will forgive the remaining balance due on your eligible Direct Loan Program loans after you have made 120 payments on those loans (after October 1, 2007) under certain repayment plans while you are employed full-time in certain public service jobs. The required 120 payments do not have to be consecutive."
With that wording, it would be very difficult to go back and try to cap the amount forgiven. Now for people taking out future loans, I wouldn't count on this program in it's current form being around for much longer...