That'd be simply too much of a liability. Yes, the authors of those books signed away rights to their stories to the publishers, and the publishers to Lucasfilm

That'd be simply too much of a liability. Yes, the authors of those books signed away rights to their stories to the publishers, and the publishers to Lucasfilm Ltd., but how well written were those contracts? Did they miss anything? Are you absolutely sure that every clause is enforceable? As there is literally a billion dollars on the line, I expect there would be no shortage of greedy lawyers calling up struggling ghost writers and struggling trying to make a case even if there isn't one. I don't know what the new trilogy will be about (except it will probably have C3PO and R2D2, and Chewbacca makes a cameo). The one thing I am absolutely certain of is that none of it will be drawn from the expanded universe.