I don't accept that our fan base not expanded much over the last few years; in fact, I'd be amazed if our fan base is not growing relatively quickly. However, the growth will largely be away from the Manchester area. Also, in the UK, the growth will be among young kids who are probably years away from being in a position to attend game at the Etihad regularly.
If City continue to improve as a team and club and keep winning titles, the demand for seats at the Etihad will continue to grow.
The club will have all sorts of data to tell them how support is growing and what demand for tickets looks like and what trend it is on. They will calculate if and when it makes sense to increase capacity.
The point does bear repeating that the empty seats at the Etihad are largely down to people not using tickets rather than lack of demand for tickets.
People that have bought season tickets are entitled to attend as and when they see fit but I do find it odd that so many people seem unwilling or unable to get someone to use their seats when they are not there. Our Kid and I have a season ticket each, I live 180 miles from MCR, he still resides in Cheshire but has to travel regularly for work so there are plenty of occasions when one or both of us cannot go but our seats are rarely empty and we now regret that we gave up a third seat a few years back. I have friends who don't even support City that would be delighted to borrow our tickets just because City are so good to watch.