This is a tricky one to answer because the question's tricky.
If, for some bizarre reason, City ceased to exist then I suspect most City fans would stop taking an active interest in going to watch football, but would watch the odd game. The minority would adopt another club, either one they've already got a soft spot for or a new one influenced by family, friends, location etc.
However - and this is a big however - had City never existed, then in all likelihood many current City fans would be rags, despite the protestations that this could and would never happen. Many of us support City because they are our home-town team. However, the argument that the rags don't play in Manchester only perpetuates because there is a rival club to make a comparison with and so would never have arisen. Therefore, it would seem pefectly natural to support the rags as the nearest major club to Manchester.
Thank God for City then!
(TSV 1860 München for me BTW.)