I think football is different with regards to 'selling out'. In a supermarket if you sell out of onions the word spreads that the place in question never had any stock left, and thus people go and shop elsewhere. In football a 'sell out' is a sign of success, and any excess, in this case - empty seats, are frowned upon by the footballing community. Sell outs over a long period of time creates demand; the domino effect of people wanting tickets in fear of missing out.
If a stadium is full of empty seats then people are more relaxed about buying, and if they're too relaxed they may not even bother buying at all. If a stadium is full week in week out then people become concerned about missing out and thus become more pro active with their ticket hunt. If you look at United, they were getting 44,000 when they started winning the league. They expanded it after selling out every match for around three seasons, with Government money of course, to 55,000 when they rebuilt the North Stand. When that sold out every match they put another tier on the East Stand (61,000), followed by another tier on the Stretford End (68,000). When that sold out every match they filled in the corners and got up to 76,000. That's a 15 year development plan sustained by continued sell outs and success on the field.
At City a good side and a bit of hope sees us selling out 47,500 and that's without winning anything and a poor top division side sees us getting about 42,000, which is pretty good no matter what anyone says. We need to fill up COMS for three years and build up an excess base of supporters who have not been for years and are finally being lured back in, along with glory hunters and tourists as well, like it or not! Then we can think about putting 6,000 on the South Stand, and when we sell out 53,500 every games for 2-3 years we can put 6,000 on the North Stand as well. So in reality we not shy off a decade before we're value for a 60,000 seater stadium - going off the successful United model.
