We all have different reasons for why The fact that you can both produce the same externally visible results for one particular test as a result of your training does not suggest that you are both in fact training the same thing.
I agree. But we have to share something in order to find our differences. Until we can get an "internal" and "external" person to do the same thing, we can't look at how they do that thing differently.
I assume when we talk about "internal" and "external" martial arts, we are still talking about them as relating to martial situations. Something martial is by it's nature, committed to achieving a result- martial dominance. Now, very few of us are soldiers or professional fighters, so being martially dominant isn't the number one objective for most of us, but when we are talking about using our bodies in martial situations, we are talking about better ways to achieve a result.
If we're not
talking about martial effectiveness on some level, then I agree, training is purely subjective, and whatever floats your boat is what you should do. However when we want to compare two "kinds" of physical movement methods, we are indeed talking about getting a result. Only through setting a goal, and analyzing the way two different methods achieve that goal can we start to discuss if there are any real differences between those methods, and how one may be more or less advantageous in different situations.