It's not entirely correct that the O-line is preventing Trevor Siemian from being a good NFL starting QB.  True, the O-line is a huge problem.  However, so is Siemian's decision making.  Whether he's throwing balls to receivers who are well covered, or scrambling into pressure, Trevor is as much of a problem as the O-line.  If opposing defenses stack the box with 7 or 8 dudes, it's up to Trevor to throw them out of that defense.  He has to get rid of the ball quicker and more accurately.  If he can't do that, then it doesn't matter who is playing on the O-line.  He's as much to blame, if not slightly more, than the O-line.