I appreciate the perspective given in a comment to the original post and I mean no disrespect to these men or their other accomplishments—which is why I note that there were no illegalities in their actions in their day.
But as Obama prepares to become our 44th President—43rd if you look at Grover Cleveland (22nd and 24th) as a single individual, and then discounting Carter, Clinton, Bush, and Bush since they are living still—there are 34 others to choose from for paper money.
Keep in mind that not everyone on our paper money was a President—Hamilton and Franklin for instance—and that other Presidents were/are on coins but not paper such as Eisenhower and Kennedy. We have only had two women on coins—Sacagawea and Susan B Anthony. The wealth (pun intended) of potential faces is such that the question should be who isn’t on the money.
The US Treasury has sought ways to expand the collectible nature of money as seen with the presidential dollar coins and the complimentary first lady series. Clearly, they recognize the value of fresh faces.
There were many great Americans who could be so honored and perhaps the best approach is to not only upgrade the money with anti-counterfeiting colors, strips, and images, but also to periodically change the images that are on it.
A final point is that real people don’t have to be on our money at all. The older dollar coins had the “Walking Liberty” and “Peace” images, and even the Eagle coins today have the personification of Liberty (same design) on them.
In this day and age of political correctness—a term I actually despise—I’m actually surprised that no-one has brought a class-action suit on behalf of a particular minority group to have this changed.