Your argument has some merit especially regarding population but if you live north or west Belfast it is just as easy to get to BFS
The 800,000 I believe is the number of return journeys quoted after FR left, ie 400,000 return flights. Prior to FR arriving at BHD ezy had three flights to BRS reduced to two, five flights to Stansted reduced to four etc. it's only one cake and the numbers are only being switched from one airport to another. Since FR left ezy have increased their flights again
When EI moves to BHD their numbers will go up and BFS down for a short time.
Again it begs the question do we need three commercial airport for NI?