re-read the last paragraph of my last post (#16). I think that has the answers you want.
In an attempt to be more specific:
A) If we define economy as oxygen used/watt, than this is indeed a fixed number for long term (>10min) efforts.
B) Your LT is not limited by your economy, because you haven't maxed out on the top of that fraction in (A). Keeping the same economy, you need to train your body to use more oxygen, and thus gain more power.
You can do this because although V02 max isn't trainable, we aren't talking about that. We are talking about V02 at LT, which is absolutely trainable. It's the reason we are here.
In addition: There is no reason to think that you are less economical than anybody else. You simply aren't tapping into as much of your total 02 consumption as you could be.
This is all fun to discuss academically, but in reality, your body only knows response to stimulus, and the stimulus that you should provide is more or less agreed upon by all reputable coaches/authors on the subject. Do base, do LT, then do super intensity. The details (should you do more or less z3 vs. z2 or z4) are arguable, but the general trend is widely accepted at this time in the sport.