Different people have different values of a '10/10'; it's not intrinsic. I only respect mine personally - if I was trying to evaluate the wider effect of Angel Beats!, it would probably be an eight, but I'd be making less concrete evaluations of effect in that case. I also don't consider 'full potential', because that would likewise only be supposition. I consider what effect the show had on me - if it left me completely satisfied, especially in the short term, it's a ten.

My view of how a show had 'realised its ideas' is just mine. I can't speak outside of my own experience. So I guess Angel Beats! does reach its 'full potential' for a number of its viewers, myself included. As for its 'full potential' in terms of its own nature as an abstract piece of art, I can't usefully comment.

As for this:

Where do we draw the line when it comes to external influences on a piece of television/film being used as some 'meta' excuse for its shortcomings.

The word 'excuse' implies some agenda behind the criticism. We can't really talk about that as critics without just getting into pointless discussions where we're only trying to defend ourselves. Rather, we need to keep looking at the show: is there anything that goes against this view of the story? I haven't mentioned a number of things that further it.

The 'meta' is another divisive area of art. Some people see the context as part of the work - some people distance them. I'm doing the former here because Angel Beats! has become famous for the kind of criticism it gets, and I haven't seen ways of relating these wider things to aspects of the show in any ways that take away from its beauty.