I guess we will never know...
It's good to know that in your mind, Hall would have put up huge numbers in Boston. Unfortunately, in Claude Julien's mind, rookies, particularly ones that need to be better defensively, must earn their TOI by showing they are not going to cost the team with defensive zone errors.
Don't see how Hall would have put up huge numbers while he was busy learning to be a complete player?
So I guess Seguin failed at getting better defensively since he consistently played... 11 or so minutes (not sure exact number) a game through every. Game. Of. The. Season? Is that what you're trying to say? Using yours or in this case "Claude Julien's" logic, rookies only get more time on the ice if they improve defensively, no? So does Seguin's constant benching mean that he didn't improve as the season went on? If that's the case then I think we all know who the better player is right now.
There must be other reasons Seguin was benched.