I think you're wrong in thinking we wouldn't fill a 70k plus stadium, even now. I've been going watching City for 40 odd years. Many of the fans I went with regularly in my early years as a supporter haven't been going for 2 decades for one reason or another, moving away, debt, work, family life etc. I sit in a group of 8 family and friends presently. If one or more can't make the game, their seat is always taken by another friend or acquaintance. I know more people that call themselves City fans who don't ever go to games, than do, and a similar number who go occasionally.
Millions of City fans exist, and the number grows by the day as our brand and success grows. If everybody I knew who were City fans went to 7 or 8 home games per season, we'd fill it no problem. Nearly 40% of our growing fan base is in the South East (taken from ticket and season card sales), you can see young kids everywhere wearing City tops, we have attracted the next younger generation already, those born over the next 5 to 10 years will also jump on the success bandwagon, just as kids did with the Rags, Chelsea, Arsenal and Liverpool in the past. The big difference now, which benefits City to no end, is that our owners vision for the future has ensured that we have already become a huge worldwide attraction to new fans (like it or lump it), and social media and the web ensures that the World hears and sees what we do as a club far quicker than the successful clubs in the past. City as a brand will overtake all other clubs within 10 years, probably far less, the fan base will grow hugely, we'll need a bigger stadium for the 'tourists', we need it now, 61000 will be just fine for a few years, but expect that to be too small in the not to distant future. I think in the meantime we will see around 65000 in the ground with some 'safe standing' added, not long after the North Stand expansion...?!