I think that it's interesting that you're willing to concede that Erin's superior familiarity with both teaching and differentiated instruction only make you willing to "concede that it may be possible to educate everyone in that middle range together" when that's not what she's saying.

I have to say that conceding that you have no evidence for the basic thesis of your assertion, let alone the assertion itself, that it's unlikely to even exist, and that someone arguing the opposite position has superior experience in this specific area and has offered evidence but still insisting that you must be right and they must be wrong looks exactly like a cop-out.