Basil,
except that Boeing has dropped the "IGW" designation in favor of the good old "ER". They now well 777-200ERs, not 200IGWs.
And as further rebuttal, consider the 767-400ER. By your logic, since the model is equipped for long-range ops from the start, there would be no need for the 'ER' designator. But not only does it have one, it's actually part of the official designation, as inscribed on the manufacturer's plate (all other 767 models are officially designated 767-200 or 767-300, with no 'ER' , even if they are 'ER' models; the 'ER' here is a marketing name, not an official designation).
Anyway, the basic point is; DON'T LOOK FOR LOGIC IN AIRLINER DESIGNATIONS. They are as much as product of fads and marketing spin as they are of engineering. When the manufacturers think a new designation will help sell the airplane, they'll apply it. When they think stressing commonality will help sell the airplane, they'll keep the old designation, perhaps with a modifier.
The 747-300 is really very little different from a 747-200 except for the stretched upper deck, but it gets a new model number. But later, the 747-400F omits the stretched upper deck but keeps the -400 model number of the basic 747-400. Why does the addition of a stretched upper deck rate a new model number, but not its deletion?
The 747-200 is basically a 747-100 with higher weights (and a few other incremental improvements), but it rates a new model number. Today, an increase in weights at best rates an 'ER' (or, for a short time, an 'IGW').
Don't look for logic, or at least don't look for a consistency of logic over time.