That might not have been the best way for me to make my point, which is just that, qualitatively, all human languages appear to be incredibly similar. I am not aware of any way to quantify this observation. I do know, however, that children who do not acquire any language during the critical period find it much more difficult to acquire a language as an adult relative to a typical adult acquiring a second language [0]. This means that a native English speaker would be realying heavily of his knowledge of English when learning Japanese (although not necessarily at a conscious level).

I tried to avoid the "apple to oranges" objection you are raising by restricting the domain to that of "potential languages", which seems like the "fair" point of reference to take. Since Japanese and English are both natural human languages, I would consider comparing them more akin to comparing red apples and green apples.

The main point I was trying to make (which admittadly got berried at the end of my post) was that "wrong" and "not useful" are two very different concepts.

[0] This is best observed in deaf children who are not exposed to sign language at a young age.