You can't sell exactly 7500 seats for every game in the belief that exactly 7500 and no more will turn to up fill every seat and be happy to sit on their own in the rubbish front seats away from friends and family members..
Just imagine if we had a modernised12000 capacity, atmospheric staduim with 8000 plus fans sitting and standing in there prefered positions amoungst their friends? How much more attractive a proposition is that?
Having extra capacity is equivalent of having extra stock, it gives you an oppertunity to sell your product.
Hope that helps.
I work in the aviation industry so let me provide you with an analogy: no airline worth its salt goes out and commits itself to the committed expenditure of additional aircraft capacity "as an opportunity to sell its product". They will only do so if and when there is a sustainable demand (load factor) for seats which outstrips existing capacity. Why should football stadia expansion be any different.
Alternatively let's just ignore the economics and put a B747 on a B737-demand route in order to give passengers more room and a greater opportunity of getting their preferred seats.
And, actually, I'm not a "KT Man" … I'd rather have your autograph in my collection, if Jim can fix it.