If Letang is traded mid-season, the return would have to be someone very similar to him, making such a deal unlikely. It could only be part of a large shake-up. And while the Pens have defensive prospects, it's impossible to say if and when they'll mature into everyday NHL players and what their ceiling is. This is a win-now team, not a develop a young core team.
Like has been suggested previously, he will probably reach a crossroads like Staal did. Sign a long-term deal or wait to be traded. There is an expectation in the organization that players should be prepared to sacrifice money to stay and win.