I agree that this is a HUUUGE step towards bringing down the president.
From what Lanny Davis said, this willingness to speak candidly is Cohen’s aim. The pundits (see MSNBC’s Maddow/O’Donnell/their lawyers/experts) also say that this first step that Cohen is taking is to establish Cohen wants to present evidence to proffer a plea deal. (I think this is correct.) What Cohen’s done so far is to lay out what Cohen knows he’s liable for - bank fraud, campaign fraud and making the president, a unindicted co-conspirator*
Apparently this Cohen deal “plea” only started to get put together about 10 days ago, and the pundits (lawyers/prosecutor types) are saying that this starting point will act as a place holder to start conversations with Cohen to really see what evidence he has on the President and others, that might eventually reduce his sentence. As Lanny Davis keeps saying, that Cohen is now liberated to say what he wants.
For Cohen, they are saying, it comes down to protecting his family. Cohen realized that T was never going to protect him, and despite Cohen putting out trial balloons for a pardon or something like that, it was never received.
And for what it’s worth, pundits are also saying we’re in the 2nd Qtr of the game. So a mini half-time VICTORY DANCE may be in order! 
*An unindicted co-conspirator, or unindicted conspirator, is a person or entity that is alleged in an indictment to have engaged in conspiracy, but who is not charged in the same indictment.
