I just did a check on the US Treasury website. They no longer print denominations over $100 so that leaves out a lot of potential people they could replace if they decided to go through with it (like Chase and Wilson - who was Chase?

). The people currently on our paper currency:
George Washington on the $1 bill
Thomas Jefferson on the $2 bill
Abraham Lincoln on the $5 bill
Alexander Hamilton on the $10 bill
Andrew Jackson on the $20 bill
Ulysses S. Grant on the $50 bill
Benjamin Franklin on the $100 bill
George Washington and Abraham Lincoln are about the best overall presidents we had so we're not going to replace them. If they had to replace any of them why not replace Grant? I don't think there are too many Grant fans around these days.
Also his administration was one of the most corrupt presidential administrations we've ever had (although Grant himself was innocent). He was a great general, but a poor administrator and selector of personnel to fill positions in government. He wasn't like Washington (good general, good administrator and good politician)
Maybe the Civil War buffs would get upset if they decided to remove him.
Let's see. Who's on our current coinage?:
Abraham Lincoln on the one-cent coin
Thomas Jefferson on the five-cent coin
Franklin D. Roosevelt on the ten-cent coin
George Washington on the quarter-dollar coin
John F. Kennedy on the half-dollar coin
Sacajawea is on the one dollar coin (which just began to be produced in early 2000).
No good choices to remove there. A lot of people still like FDR and there are still a number of people that are alive to remember him. He was a father figure to them too. You can't remove Sacajawea because that would be politically incorrect. Maybe if the coin had been in production for ten or fifteen years, but they can't remove her yet.